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/language_selector/src/language_selector.rs

@@ -19,7 +19,13 @@ use ui::{HighlightedLabel, ListItem, ListItemSpacing, prelude::*};
 use util::ResultExt;
 use workspace::{ModalView, Workspace};
 
-actions!(language_selector, [Toggle]);
+actions!(
+    language_selector,
+    [
+        /// Toggles the language selector modal.
+        Toggle
+    ]
+);
 
 pub fn init(cx: &mut App) {
     cx.observe_new(LanguageSelector::register).detach();