Mark Hooks::runner() as @internal

It returns a HookRunner which is documented as "It is intended for use
within MediaWiki core only. Extensions that need a hook runner should
create one for the hooks they need to run."

Since the returned object is only for core, the method should only
be called in core.

Change-Id: I6dfc605a37a787d1f4cdb16b8f1831c338619776

includes/Hooks.php

@@ -166,6 +166,7 @@ class Hooks {
 	 * protected method.
 	 *
 	 * @since 1.35
+	 * @internal because HookRunner is intended for core only, see documentation on that class
 	 *
 	 * @return HookRunner
 	 */