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/git_ui/src/git_ui.rs

@@ -31,7 +31,13 @@ pub mod project_diff;
 pub(crate) mod remote_output;
 pub mod repository_selector;
 
-actions!(git, [ResetOnboarding]);
+actions!(
+    git,
+    [
+        /// Resets the git onboarding state to show the tutorial again.
+        ResetOnboarding
+    ]
+);
 
 pub fn init(cx: &mut App) {
     GitPanelSettings::register(cx);