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/zeta.rs

@@ -72,7 +72,13 @@ const MAX_EVENT_TOKENS: usize = 500;
 /// Maximum number of events to track.
 const MAX_EVENT_COUNT: usize = 16;
 
-actions!(edit_prediction, [ClearHistory]);
+actions!(
+    edit_prediction,
+    [
+        /// Clears the edit prediction history.
+        ClearHistory
+    ]
+);
 
 #[derive(Copy, Clone, Default, Debug, PartialEq, Eq, Hash)]
 pub struct InlineCompletionId(Uuid);