Docs: Further corrections and improvements to various inline docblocks.

See #49572

git-svn-id: https://develop.svn.wordpress.org/trunk@48576 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/comment.php

@@ -21,7 +21,7 @@
  * @param string $comment_author Author of the comment.
  * @param string $comment_date   Date of the comment.
  * @param string $timezone       Timezone. Accepts 'blog' or 'gmt'. Default 'blog'.
- * @return mixed Comment post ID on success.
+ * @return string|null Comment post ID on success.
  */
 function comment_exists( $comment_author, $comment_date, $timezone = 'blog' ) {
 	global $wpdb;