Mark $single parameter of get_post_meta() as optional.

props netweb.
fixes #28708.

git-svn-id: https://develop.svn.wordpress.org/trunk@28954 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -1813,7 +1813,7 @@ function delete_post_meta($post_id, $meta_key, $meta_value = '') {
  *
  * @param int $post_id Post ID.
  * @param string $key Optional. The meta key to retrieve. By default, returns data for all keys.
- * @param bool $single Whether to return a single value.
+ * @param bool $single Optional. Whether to return a single value.
  * @return mixed Will be an array if $single is false. Will be value of meta data field if $single
  *  is true.
  */