Docs: Use object shape notation for return type

Apply @westonruter's suggestion to use object shape
notation for wp_get_media_library_months_with_files()
return type, with string types matching the actual
wpdb query output.

Author: apermo

src/wp-includes/media.php

@@ -5157,7 +5157,7 @@ function wp_enqueue_media( $args = array() ) {
  *
  * @global wpdb $wpdb WordPress database abstraction object.
  *
- * @return stdClass[] Array of objects with `month` and `year` properties.
+ * @return array<int, object{year: string, month: string}> Array of objects with `month` and `year` properties.
  */
 function wp_get_media_library_months_with_files(): array {
 	global $wpdb;