Docs: Improve accuracy of @return type and description for get_post_custom().

Developed in WordPress#10996

Follow-up to [52795].

Props ravikhadka, westonruter, swissspidy, roytanck.
See #64224, #55249.
Fixes #60646.


git-svn-id: https://develop.svn.wordpress.org/trunk@61718 602fd350-edb4-49c9-b593-d223f7449a82

Author: westonruter

src/wp-includes/post.php

@@ -2828,9 +2828,11 @@ function unregister_post_meta( $post_type, $meta_key ) {
  * @since 1.2.0
  *
  * @param int $post_id Optional. Post ID. Default is the ID of the global `$post`.
- * @return mixed An array of values.
- *               False for an invalid `$post_id` (non-numeric, zero, or negative value).
- *               An empty string if a valid but non-existing post ID is passed.
+ * @return array<string, array<int, string>>|false Array of post meta values keyed by meta key, or false on failure.
+ *                                                 Post meta values will always be strings, even for values which would
+ *                                                 otherwise be retrieved individually as arrays or objects via
+ *                                                 {@see get_post_meta()}. An empty array is returned if the post has
+ *                                                 no post meta.
  */
 function get_post_custom( $post_id = 0 ) {
 	$post_id = absint( $post_id );