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/action.rs

@@ -225,6 +225,7 @@ pub(crate) struct ActionRegistry {
     all_names: Vec<&'static str>, // So we can return a static slice.
     deprecated_aliases: HashMap<&'static str, &'static str>, // deprecated name -> preferred name
     deprecation_messages: HashMap<&'static str, &'static str>, // action name -> deprecation message
+    documentation: HashMap<&'static str, &'static str>, // action name -> documentation
 }
 
 impl Default for ActionRegistry {
@@ -232,6 +233,7 @@ impl Default for ActionRegistry {
         let mut this = ActionRegistry {
             by_name: Default::default(),
             names_by_type_id: Default::default(),
+            documentation: Default::default(),
             all_names: Default::default(),
             deprecated_aliases: Default::default(),
             deprecation_messages: Default::default(),
@@ -327,6 +329,9 @@ impl ActionRegistry {
         if let Some(deprecation_msg) = action.deprecation_message {
             self.deprecation_messages.insert(name, deprecation_msg);
         }
+        if let Some(documentation) = action.documentation {
+            self.documentation.insert(name, documentation);
+        }
     }
 
     /// Construct an action based on its name and optional JSON parameters sourced from the keymap.
@@ -388,6 +393,10 @@ impl ActionRegistry {
     pub fn deprecation_messages(&self) -> &HashMap<&'static str, &'static str> {
         &self.deprecation_messages
     }
+
+    pub fn documentation(&self) -> &HashMap<&'static str, &'static str> {
+        &self.documentation
+    }
 }
 
 /// Generate a list of all the registered actions.

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>(