Docs: Reduce confusion in @since MU notationss by indicating the WP version (3.0.0) the code was merged into core while retaining the original context.

Props sathyapulse.
Fixes #41509.


git-svn-id: https://develop.svn.wordpress.org/trunk@41200 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/menu-header.php

@@ -31,7 +31,7 @@ global $menu, $submenu, $parent_file, $submenu_file;
  *
  * Allows plugins to move sub-menu items around.
  *
- * @since MU
+ * @since MU (3.0.0)
  *
  * @param string $parent_file The parent file.
  */