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/picker/src/picker.rs

@@ -34,7 +34,13 @@ pub enum Direction {
     Down,
 }
 
-actions!(picker, [ConfirmCompletion]);
+actions!(
+    picker,
+    [
+        /// Confirms the selected completion in the picker.
+        ConfirmCompletion
+    ]
+);
 
 /// ConfirmInput is an alternative editor action which - instead of selecting active picker entry - treats pickers editor input literally,
 /// performing some kind of action on it.