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/search/src/project_search.rs

@@ -47,7 +47,16 @@ use workspace::{
 
 actions!(
     project_search,
-    [SearchInNew, ToggleFocus, NextField, ToggleFilters]
+    [
+        /// Searches in a new project search tab.
+        SearchInNew,
+        /// Toggles focus between the search bar and the search results.
+        ToggleFocus,
+        /// Moves to the next input field.
+        NextField,
+        /// Toggles the search filters panel.
+        ToggleFilters
+    ]
 );
 
 #[derive(Default)]