Docs: Add missing `@since` entry for `Walker::unset_children()`.

Props chiragpatel.
Fixes #39506.

git-svn-id: https://develop.svn.wordpress.org/trunk@39741 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-walker.php

@@ -404,6 +404,8 @@ class Walker {
 	/**
 	 * Unset all the children for a given top level element.
 	 *
+	 * @since 2.7.0
+	 *
 	 * @param object $e
 	 * @param array $children_elements
 	 */