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/zeta/src/init.rs

@@ -10,7 +10,15 @@ use workspace::Workspace;
 
 use crate::{RateCompletionModal, onboarding_modal::ZedPredictModal};
 
-actions!(edit_prediction, [ResetOnboarding, RateCompletions]);
+actions!(
+    edit_prediction,
+    [
+        /// Resets the edit prediction onboarding state.
+        ResetOnboarding,
+        /// Opens the rate completions modal.
+        RateCompletions
+    ]
+);
 
 pub fn init(cx: &mut App) {
     cx.observe_new(move |workspace: &mut Workspace, _, _cx| {