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/change_list.rs

@@ -3,7 +3,15 @@ use gpui::{Context, Window, actions};
 
 use crate::{Vim, state::Mode};
 
-actions!(vim, [ChangeListOlder, ChangeListNewer]);
+actions!(
+    vim,
+    [
+        /// Navigates to an older position in the change list.
+        ChangeListOlder,
+        /// Navigates to a newer position in the change list.
+        ChangeListNewer
+    ]
+);
 
 pub(crate) fn register(editor: &mut Editor, cx: &mut Context<Vim>) {
     Vim::action(editor, cx, |vim, _: &ChangeListOlder, window, cx| {