Docs: Improve the `@return` description of `network_domain_check()`.

Props ishitaka, vaishalipanchal.
Fixes #45403.



git-svn-id: https://develop.svn.wordpress.org/trunk@44460 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/network.php

@@ -14,7 +14,7 @@
  *
  * @global wpdb $wpdb WordPress database abstraction object.
  *
- * @return Whether a network exists.
+ * @return string|false Base domain if network exists, otherwise false.
  */
 function network_domain_check() {
 	global $wpdb;