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/buffer_search.rs

@@ -46,6 +46,7 @@ use registrar::{ForDeployed, ForDismissed, SearchActionsRegistrar, WithResults};
 
 const MAX_BUFFER_SEARCH_HISTORY_SIZE: usize = 50;
 
+/// Opens the buffer search interface with the specified configuration.
 #[derive(PartialEq, Clone, Deserialize, JsonSchema, Action)]
 #[action(namespace = buffer_search)]
 #[serde(deny_unknown_fields)]
@@ -58,7 +59,17 @@ pub struct Deploy {
     pub selection_search_enabled: bool,
 }
 
-actions!(buffer_search, [DeployReplace, Dismiss, FocusEditor]);
+actions!(
+    buffer_search,
+    [
+        /// Deploys the search and replace interface.
+        DeployReplace,
+        /// Dismisses the search bar.
+        Dismiss,
+        /// Focuses back on the editor.
+        FocusEditor
+    ]
+);
 
 impl Deploy {
     pub fn find() -> Self {