Docs: Add $object_subtype param documentation for sanitize_meta().

Props timothyblynjacobs.

See #38323.


git-svn-id: https://develop.svn.wordpress.org/trunk@43521 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/meta.php

@@ -979,6 +979,7 @@ function is_protected_meta( $meta_key, $meta_type = null ) {
  * @param string $meta_key       Meta key.
  * @param mixed  $meta_value     Meta value to sanitize.
  * @param string $object_type    Type of object the meta is registered to.
+ * @param string $object_subtype Optional. The subtype of the object type.
  *
  * @return mixed Sanitized $meta_value.
  */