minor #20425 [Form] URLType : explain null value for default_protocol (alexislefebvre)

This PR was submitted for the 5.4 branch but it was merged into the 6.4 branch instead.

Discussion
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[Form] URLType : explain `null` value for `default_protocol`

Sources:

- symfony/symfony#30635
- symfony/symfony#50922

Commits
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0b521e7 URLType: explain `null` value for `default_protocol`

Author: javiereguiluz

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.