REST API: Some documentation and test improvements for update_post_parent_caches():

* Make the function description more specific, for consistency with `update_post_author_caches()`.
* Add missing `@covers` tags for the unit test.

Follow-up to [53506].

See #55593.
Built from https://develop.svn.wordpress.org/trunk@53507


git-svn-id: http://core.svn.wordpress.org/trunk@53096 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/post.php

@@ -7490,16 +7490,17 @@ function update_post_author_caches( $posts ) {
 }
 
 /**
- * Prime post parent post caches.
+ * Updates parent post caches for a list of post objects.
  *
  * @since 6.1.0
  *
- * @param WP_Post[] $posts Array of Post objects.
+ * @param WP_Post[] $posts Array of post objects.
  */
 function update_post_parent_caches( $posts ) {
 	$parent_ids = wp_list_pluck( $posts, 'post_parent' );
 	$parent_ids = array_map( 'absint', $parent_ids );
 	$parent_ids = array_unique( array_filter( $parent_ids ) );
+
 	if ( ! empty( $parent_ids ) ) {
 		_prime_post_caches( $parent_ids, false );
 	}