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/terminal_view/src/terminal_panel.rs

@@ -46,7 +46,13 @@ use zed_actions::assistant::InlineAssist;
 
 const TERMINAL_PANEL_KEY: &str = "TerminalPanel";
 
-actions!(terminal_panel, [ToggleFocus]);
+actions!(
+    terminal_panel,
+    [
+        /// Toggles focus on the terminal panel.
+        ToggleFocus
+    ]
+);
 
 pub fn init(cx: &mut App) {
     cx.observe_new(