annotations: channel rule name & mark experimental

Updates the name used for the channel rule that enabled mutable messages
functionality to "Advanced message features" per ADR-144.

Marks the annotations feature as Experimental and adds the appropriate
caveats to the channel rule documentation.

Author: mschristensen

content/channels/index.textile

@@ -190,20 +190,20 @@ Channel rules can be used to enforce settings for specific channels, or channel
 
 The channel rules related to message storage are:
 
-- Persist last message := if enabled, the very last message published on a channel will be stored for a year. This message is retrievable using "rewind":/docs/channels/options/rewind by attaching to the channel with @rewind=1@. If you send multiple messages in a single protocol message, for example calling @publish()@ with an array of messages, you would receive all of them as one message. Be aware that presence messages are not stored and that messages stored in this manner are not accessible using "history":/docs/storage-history/history. Note that for each message stored using this rule, an additional message is deducted from your monthly allocation.
-- Persist all messages := if enabled, all messages published on a channel will be stored according to the storage rules for your account. This is 24 hours for free accounts and 72 hours for paid accounts. Messages stored in this manner are accessible using "history":/docs/storage-history/history. Note that for each message stored using this rule, an additional message is deducted from your monthly allocation.
+- Persist last message := If enabled, the very last message published on a channel will be stored for a year. This message is retrievable using "rewind":/docs/channels/options/rewind by attaching to the channel with @rewind=1@. If you send multiple messages in a single protocol message, for example calling @publish()@ with an array of messages, you would receive all of them as one message. Be aware that presence messages are not stored and that messages stored in this manner are not accessible using "history":/docs/storage-history/history. Note that for each message stored using this rule, an additional message is deducted from your monthly allocation.
+- Persist all messages := If enabled, all messages published on a channel will be stored according to the storage rules for your account. This is 24 hours for free accounts and 72 hours for paid accounts. Messages stored in this manner are accessible using "history":/docs/storage-history/history. Note that for each message stored using this rule, an additional message is deducted from your monthly allocation.
 
 The channel rules related to security and client identity are:
 
-- Identified := if enabled, clients will not be permitted to use (including to attach, publish, or subscribe) matching channels unless they are "identified":/docs/auth/identified-clients (they have an assigned client ID). Anonymous clients are not permitted to join these channels. Find out more about "authenticated and identified clients":/docs/auth/identified-clients.
-- TLS only := if enabled, only clients who have connected to Ably over TLS will be allowed to use matching channels. By default all of Ably's client libraries use TLS when communicating with Ably over REST or when using our Realtime transports such as Websockets.
+- Identified := If enabled, clients will not be permitted to use (including to attach, publish, or subscribe) matching channels unless they are "identified":/docs/auth/identified-clients (they have an assigned client ID). Anonymous clients are not permitted to join these channels. Find out more about "authenticated and identified clients":/docs/auth/identified-clients.
+- TLS only := If enabled, only clients who have connected to Ably over TLS will be allowed to use matching channels. By default all of Ably's client libraries use TLS when communicating with Ably over REST or when using our Realtime transports such as Websockets.
 
 The channel rules related to enabling features are:
 
-- Push notifications enabled := If checked, publishing messages with a push payload in the @extras@ field is permitted. This triggers the delivery of a "Push Notification":/docs/push to devices registered for push on the channel.
-- Server-side batching := if enabled, messages are grouped into batches before being sent to subscribers. "Server-side batching":/docs/messages/batch#server-side reduces the overall message count, lowers costs, and mitigates the risk of hitting rate limits during high-throughput scenarios.
-- Message conflation := if enabled, messages are aggregated over a set period of time and evaluated against a conflation key. All but the latest message for each conflation key value will be discarded, and the resulting message, or messages, will be delivered to subscribers as a single batch once the period of time elapses. "Message conflation":/docs/messages#conflation reduces costs in high-throughput scenarios by removing redundant and outdated messages.
-- Mutable messages := if enabled, allows use of message "annotations":/docs/annotations .
+- Push notifications enabled := If enabled, publishing messages with a push payload in the @extras@ field is permitted. This triggers the delivery of a "Push Notification":/docs/push to devices registered for push on the channel.
+- Server-side batching := If enabled, messages are grouped into batches before being sent to subscribers. "Server-side batching":/docs/messages/batch#server-side reduces the overall message count, lowers costs, and mitigates the risk of hitting rate limits during high-throughput scenarios.
+- Message conflation := If enabled, messages are aggregated over a set period of time and evaluated against a conflation key. All but the latest message for each conflation key value will be discarded, and the resulting message, or messages, will be delivered to subscribers as a single batch once the period of time elapses. "Message conflation":/docs/messages#conflation reduces costs in high-throughput scenarios by removing redundant and outdated messages.
+- Advanced message features := If enabled, advanced messaging features including message "annotations":/docs/annotations are available on the channel. Note that advanced messaging is currently Experimental and its features are still in development and subject to rapid change. Currently, when advanced messaging is enabled, messages are "persisted":/docs/storage-history/storage#all-message-persistence by default, and "continuous history":/docs/storage-history/history#continuous-history features are not currently supported.
 
 <!-- DASHBOARD-INSTRUCTION: Setting a channel rule -->
 To set a channel rule in the Ably dashboard:

src/pages/docs/messages/annotations.mdx

@@ -6,6 +6,10 @@ languages:
   - nodejs
 ---
 
+<Aside data-type='experimental'>
+Message Annotations is currently Experimental. Its features are still in development and subject to rapid change.
+</Aside>
+
 Message annotations allow clients to add metadata to existing messages on a channel. You can use annotations to implement features like:
 
 - **Message reactions** - Add emoji reactions (👍, ❤️, 😂) to messages
@@ -18,13 +22,17 @@ When clients publish or delete annotations, Ably automatically creates a [summar
 
 ## Enable annotations <a id="enable"/>
 
-Annotations can be enabled for a channel or channel namespace with the *Annotations* channel rule.
+Annotations can be enabled for a channel or channel namespace with the *Advanced message features* channel rule.
+
+<Aside data-type='important'>
+Note that when advanced messaging is enabled, messages are [persisted](/docs/storage-history/storage#all-message-persistence) by default, and [continuous history](/docs/storage-history/history#continuous-history) features are not currently supported.
+</Aside>
 
 1. On your [dashboard](https://ably.com/accounts/any), select one of your apps.
 2. Go to **Settings**.
 3. Under [channel rules](/docs/channels#rules), click **Add new rule**.
 4. Enter the channel name or channel namespace on which to enable message annotations.
-5. Check **Annotations** to enable message annotations.
+5. Check **Advanced message features** to enable message annotations.
 6. Click **Create channel rule** to save.