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/rewrap.rs

@@ -4,7 +4,13 @@ use editor::{Bias, Editor, RewrapOptions, SelectionEffects, display_map::ToDispl
 use gpui::{Context, Window, actions};
 use language::SelectionGoal;
 
-actions!(vim, [Rewrap]);
+actions!(
+    vim,
+    [
+        /// Rewraps the selected text to fit within the line width.
+        Rewrap
+    ]
+);
 
 pub(crate) fn register(editor: &mut Editor, cx: &mut Context<Vim>) {
     Vim::action(editor, cx, |vim, _: &Rewrap, window, cx| {