Docs: Improve documentation for the $prev_value parameter and return result of various metadata update functions.

Props stevenlinx.
Fixes #50502. See #49572.

git-svn-id: https://develop.svn.wordpress.org/trunk@48214 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/comment.php

@@ -517,9 +517,11 @@ function get_comment_meta( $comment_id, $key = '', $single = false ) {
  * @param string $meta_key   Metadata key.
  * @param mixed  $meta_value Metadata value. Must be serializable if non-scalar.
  * @param mixed  $prev_value Optional. Previous value to check before updating.
- *                           Default empty.
+ *                           If specified, only update existing metadata entries with
+ *                           this value. Otherwise, update all entries. Default empty.
  * @return int|bool Meta ID if the key didn't exist, true on successful update,
- *                  false on failure.
+ *                  false on failure or if the value passed to the function
+ *                  is the same as the one that is already in the database.
  */
 function update_comment_meta( $comment_id, $meta_key, $meta_value, $prev_value = '' ) {
 	return update_metadata( 'comment', $comment_id, $meta_key, $meta_value, $prev_value );