Multisite: Document all return types in get_sites().

Adds more complete documentation to `get_sites()` and the corresponding methods in `WP_Site_Query`.

Fixes #41789.


git-svn-id: https://develop.svn.wordpress.org/trunk@41912 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/ms-blogs.php

@@ -636,7 +636,8 @@ function update_site_cache( $sites ) {
  *                                           Default empty array.
  *     @type bool         $update_site_cache Whether to prime the cache for found sites. Default true.
  * }
- * @return array|int List of sites, or number of sites when 'count' is passed as a query var.
+ * @return array|int List of WP_Site objects, a list of site ids when 'fields' is set to 'ids',
+ *                   or the number of sites when 'count' is passed as a query var.
  */
 function get_sites( $args = array() ) {
 	$query = new WP_Site_Query();