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/helix.rs

@@ -6,7 +6,13 @@ use text::SelectionGoal;
 
 use crate::{Vim, motion::Motion, state::Mode};
 
-actions!(vim, [HelixNormalAfter]);
+actions!(
+    vim,
+    [
+        /// Switches to normal mode after the cursor (Helix-style).
+        HelixNormalAfter
+    ]
+);
 
 pub fn register(editor: &mut Editor, cx: &mut Context<Vim>) {
     Vim::action(editor, cx, Vim::helix_normal_after);