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/insert.rs

@@ -5,7 +5,15 @@ use language::SelectionGoal;
 use settings::Settings;
 use vim_mode_setting::HelixModeSetting;
 
-actions!(vim, [NormalBefore, TemporaryNormal]);
+actions!(
+    vim,
+    [
+        /// Switches to normal mode with cursor positioned before the current character.
+        NormalBefore,
+        /// Temporarily switches to normal mode for one command.
+        TemporaryNormal
+    ]
+);
 
 pub fn register(editor: &mut Editor, cx: &mut Context<Vim>) {
     Vim::action(editor, cx, Vim::normal_before);