Refactor BSS key and cross-browser extensions docs to clarify manifest version compatibility #2152
Description
@dotproto I'm wondering whether we need to re-factor this content to make clear that this applies to both manifest V2 and V3 (where you have to specify an ID and therefore include browser_specific_settings). In addition, we should probably mention this in the cross browser discussion e.g. https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Build_a_cross_browser_extension#manifest_keys
Originally posted by @rebloor in #2129 (review)