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/project/src/context_server_store.rs

@@ -21,7 +21,13 @@ pub fn init(cx: &mut App) {
     extension::init(cx);
 }
 
-actions!(context_server, [Restart]);
+actions!(
+    context_server,
+    [
+        /// Restarts the context server.
+        Restart
+    ]
+);
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub enum ContextServerStatus {