diff --git a/wp-includes/comment.php b/wp-includes/comment.php
index 59059cf19..cd51cb74f 100644
--- a/wp-includes/comment.php
+++ b/wp-includes/comment.php
@@ -538,7 +538,7 @@ function delete_comment_meta($comment_id, $meta_key, $meta_value = '') {
  * @link http://codex.wordpress.org/Function_Reference/get_comment_meta
  *
  * @param int $comment_id Comment ID.
- * @param string $key The meta key to retrieve.
+ * @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.
  * @return mixed Will be an array if $single is false. Will be value of meta data field if $single
  *  is true.
diff --git a/wp-includes/post.php b/wp-includes/post.php
index 085747a93..e6713ac98 100644
--- a/wp-includes/post.php
+++ b/wp-includes/post.php
@@ -1462,7 +1462,7 @@ function delete_post_meta($post_id, $meta_key, $meta_value = '') {
  * @link http://codex.wordpress.org/Function_Reference/get_post_meta
  *
  * @param int $post_id Post ID.
- * @param string $key The meta key to retrieve.
+ * @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.
  * @return mixed Will be an array if $single is false. Will be value of meta data field if $single
  *  is true.
diff --git a/wp-includes/user.php b/wp-includes/user.php
index e81686564..aec03d671 100644
--- a/wp-includes/user.php
+++ b/wp-includes/user.php
@@ -798,7 +798,7 @@ function delete_user_meta($user_id, $meta_key, $meta_value = '') {
  * @link http://codex.wordpress.org/Function_Reference/get_user_meta
  *
  * @param int $user_id Post ID.
- * @param string $key The meta key to retrieve.
+ * @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.
  * @return mixed Will be an array if $single is false. Will be value of meta data field if $single
  *  is true.