keymap_ui: Hover tooltip for action documentation (#33862)

Closes #ISSUE

Show the documentation for an action when hovered. As a bonus, also show
the humanized command palette name!

Release Notes:

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

crates/gpui/src/app.rs

@@ -1403,11 +1403,16 @@ impl App {
         self.actions.deprecated_aliases()
     }
 
-    /// Get a list of all action deprecation messages.
+    /// Get a map from an action name to the deprecation messages.
     pub fn action_deprecation_messages(&self) -> &HashMap<&'static str, &'static str> {
         self.actions.deprecation_messages()
     }
 
+    /// Get a map from an action name to the documentation.
+    pub fn action_documentation(&self) -> &HashMap<&'static str, &'static str> {
+        self.actions.documentation()
+    }
+
     /// Register a callback to be invoked when the application is about to quit.
     /// It is not possible to cancel the quit event at this point.
     pub fn on_app_quit<Fut>(