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/extension_host/src/extension_host.rs

@@ -178,7 +178,13 @@ pub struct ExtensionIndexLanguageEntry {
     pub grammar: Option<Arc<str>>,
 }
 
-actions!(zed, [ReloadExtensions]);
+actions!(
+    zed,
+    [
+        /// Reloads all installed extensions.
+        ReloadExtensions
+    ]
+);
 
 pub fn init(
     extension_host_proxy: Arc<ExtensionHostProxy>,