URLType: explain null value for default_protocol

Author: alexislefebvre

reference/forms/types/url.rst

@@ -27,6 +27,8 @@ Field Options
 
 **type**: ``string`` **default**: ``http``
 
+Set the value as ``null`` to render the field as ``<input type="url"/>``.
+
 If a value is submitted that doesn't begin with some protocol (e.g. ``http://``,
 ``ftp://``, etc), this protocol will be prepended to the string when
 the data is submitted to the form.