Docs: Various improvements and corrections to inline docs.

See #55646


git-svn-id: https://develop.svn.wordpress.org/trunk@54267 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -7978,7 +7978,7 @@ function wp_cache_set_posts_last_changed() {
  * @global wpdb $wpdb WordPress database abstraction object.
  *
  * @param string $type
- * @return mixed
+ * @return string[] An array of MIME types.
  */
 function get_available_post_mime_types( $type = 'attachment' ) {
 	global $wpdb;