Query: Some documentation and test improvements for update_post_author_caches():

* Make the descriptions for `update_post_author_caches()` and `update_post_caches()` more specific.
* Move the unit test into its own file, for consistency with `update_post_cache()` tests. This also allows for using shared fixtures in case more tests are added in the future.

Follow-up to [53482].

See #55716.

git-svn-id: https://develop.svn.wordpress.org/trunk@53483 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/pluggable.php

@@ -143,6 +143,7 @@ if ( ! function_exists( 'cache_users' ) ) :
 		$list = implode( ',', $clean );
 
 		$users = $wpdb->get_results( "SELECT * FROM $wpdb->users WHERE ID IN ($list)" );
+
 		foreach ( $users as $user ) {
 			update_user_caches( $user );
 		}