Proto/AI: fix description of RPCs

From #7262

• RPC: current_yearly_rate_exact and current_yearly_rate_details's descriptions should say that the returned value is in percent
• RPC: expected_issuance's description should say that the returned value is in µtez
• RPC: current_yearly_rate* descriptions should state that the value only includes participation rewards (does not include liquidity baking)

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR