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/inline_completion_button/src/inline_completion_button.rs

@@ -37,7 +37,13 @@ use zed_actions::OpenBrowser;
 use zed_llm_client::UsageLimit;
 use zeta::RateCompletions;
 
-actions!(edit_prediction, [ToggleMenu]);
+actions!(
+    edit_prediction,
+    [
+        /// Toggles the inline completion menu.
+        ToggleMenu
+    ]
+);
 
 const COPILOT_SETTINGS_URL: &str = "https://github.com/settings/copilot";