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/snippets_ui/src/snippets_ui.rs

@@ -54,7 +54,15 @@ impl From<ScopeFileName> for ScopeName {
     }
 }
 
-actions!(snippets, [ConfigureSnippets, OpenFolder]);
+actions!(
+    snippets,
+    [
+        /// Opens the snippets configuration file.
+        ConfigureSnippets,
+        /// Opens the snippets folder in the file manager.
+        OpenFolder
+    ]
+);
 
 pub fn init(cx: &mut App) {
     cx.observe_new(register).detach();