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/src/auto_update.rs

@@ -28,7 +28,17 @@ use workspace::Workspace;
 const SHOULD_SHOW_UPDATE_NOTIFICATION_KEY: &str = "auto-updater-should-show-updated-notification";
 const POLL_INTERVAL: Duration = Duration::from_secs(60 * 60);
 
-actions!(auto_update, [Check, DismissErrorMessage, ViewReleaseNotes,]);
+actions!(
+    auto_update,
+    [
+        /// Checks for available updates.
+        Check,
+        /// Dismisses the update error message.
+        DismissErrorMessage,
+        /// Opens the release notes for the current version.
+        ViewReleaseNotes,
+    ]
+);
 
 #[derive(Serialize)]
 struct UpdateRequestBody {