support parsing of docs parameter in account, argument, field directives

[rustopian]

Problem

A docs attribute (akin to Anchor's desc) is not handled by the rust attribute parsing, resulting in no account doc comments in generated JS clients.

Solution

Added support for an optional docs parameter in the #[codama(account(...))] directive. The parameter accepts a string that gets converted to the docs array field on InstructionAccountNode.

Example usage:

#[codama(account(name = "stake", writable, docs = "The stake account to operate on"))]

Alternatives considered

• Calling the param desc was considered, since Anchor uses this in its IDLs. However, the codama codebase already expresses a preference for this to be an array of strings named docs.
• Parsing directly from /// doc comments typically used for instruction definitions was considered, but this is brittle.

[rustopian]

@lorisleiva have been using this tweak locally, open to the alternatives tho

[lorisleiva]

Perfection, thank you! I think we can do the same for the argument and field attributes now that we have Docs::from_meta. ☺️