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/auto_update_ui/src/auto_update_ui.rs

@@ -12,7 +12,13 @@ use workspace::Workspace;
 use workspace::notifications::simple_message_notification::MessageNotification;
 use workspace::notifications::{NotificationId, show_app_notification};
 
-actions!(auto_update, [ViewReleaseNotesLocally]);
+actions!(
+    auto_update,
+    [
+        /// Opens release notes in the browser for the current version.
+        ViewReleaseNotesLocally
+    ]
+);
 
 pub fn init(cx: &mut App) {
     notify_if_app_was_updated(cx);