gpui: Store action documentation (#33809)

Closes #ISSUE

Adds a new `documentation` method to actions, that is extracted from doc
comments when using the `actions!` or derive macros.

Additionally, this PR adds doc comments to as many action definitions in
Zed as possible.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/copilot/src/copilot.rs

@@ -46,11 +46,17 @@ pub use crate::sign_in::{CopilotCodeVerification, initiate_sign_in, reinstall_an
 actions!(
     copilot,
     [
+        /// Requests a code completion suggestion from Copilot.
         Suggest,
+        /// Cycles to the next Copilot suggestion.
         NextSuggestion,
+        /// Cycles to the previous Copilot suggestion.
         PreviousSuggestion,
+        /// Reinstalls the Copilot language server.
         Reinstall,
+        /// Signs in to GitHub Copilot.
         SignIn,
+        /// Signs out of GitHub Copilot.
         SignOut
     ]
 );