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/vim/src/normal/repeat.rs

@@ -11,7 +11,19 @@ use editor::Editor;
 use gpui::{Action, App, Context, Window, actions};
 use workspace::Workspace;
 
-actions!(vim, [Repeat, EndRepeat, ToggleRecord, ReplayLastRecording]);
+actions!(
+    vim,
+    [
+        /// Repeats the last change.
+        Repeat,
+        /// Ends the repeat recording.
+        EndRepeat,
+        /// Toggles macro recording.
+        ToggleRecord,
+        /// Replays the last recorded macro.
+        ReplayLastRecording
+    ]
+);
 
 fn should_replay(action: &dyn Action) -> bool {
     // skip so that we don't leave the character palette open