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/search.rs

@@ -23,19 +23,35 @@ pub fn init(cx: &mut App) {
 actions!(
     search,
     [
+        /// Focuses on the search input field.
         FocusSearch,
+        /// Toggles whole word matching.
         ToggleWholeWord,
+        /// Toggles case-sensitive search.
         ToggleCaseSensitive,
+        /// Toggles searching in ignored files.
         ToggleIncludeIgnored,
+        /// Toggles regular expression mode.
         ToggleRegex,
+        /// Toggles the replace interface.
         ToggleReplace,
+        /// Toggles searching within selection only.
         ToggleSelection,
+        /// Selects the next search match.
         SelectNextMatch,
+        /// Selects the previous search match.
         SelectPreviousMatch,
+        /// Selects all search matches.
         SelectAllMatches,
+        /// Cycles through search modes.
+        CycleMode,
+        /// Navigates to the next query in search history.
         NextHistoryQuery,
+        /// Navigates to the previous query in search history.
         PreviousHistoryQuery,
+        /// Replaces all matches.
         ReplaceAll,
+        /// Replaces the next match.
         ReplaceNext,
     ]
 );