Docs: Add missing documentation for the locale meta field.

The user’s locale is passed in the `$meta` parameter for `insert_user_meta`. Added in [38705].

Props tmatsuur.
Fixes #47423.

git-svn-id: https://develop.svn.wordpress.org/trunk@45479 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/user.php

@@ -1808,6 +1808,7 @@ function wp_insert_user( $userdata ) {
 	 *                                          not forced.
 	 *     @type bool     $show_admin_bar_front Whether to show the admin bar on the front end for the user.
 	 *                                          Default true.
+	 *     @type string   $locale               User's locale. Default empty.
 	 * }
 	 * @param WP_User $user   User object.
 	 * @param bool    $update Whether the user is being updated rather than created.