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/title_bar/src/collab.rs

@@ -11,7 +11,17 @@ use workspace::notifications::DetachAndPromptErr;
 
 use crate::TitleBar;
 
-actions!(collab, [ToggleScreenSharing, ToggleMute, ToggleDeafen]);
+actions!(
+    collab,
+    [
+        /// Toggles screen sharing on or off.
+        ToggleScreenSharing,
+        /// Toggles microphone mute.
+        ToggleMute,
+        /// Toggles deafen mode (mute both microphone and speakers).
+        ToggleDeafen
+    ]
+);
 
 fn toggle_screen_sharing(_: &ToggleScreenSharing, window: &mut Window, cx: &mut App) {
     let call = ActiveCall::global(cx).read(cx);