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/client/src/client.rs

@@ -81,7 +81,17 @@ pub const INITIAL_RECONNECTION_DELAY: Duration = Duration::from_millis(500);
 pub const MAX_RECONNECTION_DELAY: Duration = Duration::from_secs(10);
 pub const CONNECTION_TIMEOUT: Duration = Duration::from_secs(20);
 
-actions!(client, [SignIn, SignOut, Reconnect]);
+actions!(
+    client,
+    [
+        /// Signs in to Zed account.
+        SignIn,
+        /// Signs out of Zed account.
+        SignOut,
+        /// Reconnects to the collaboration server.
+        Reconnect
+    ]
+);
 
 #[derive(Clone, Default, Serialize, Deserialize, JsonSchema)]
 pub struct ClientSettingsContent {