Fixed #23075 -- Added documentation on novalidate attribute and made it default for admin

Thanks to sehmaschine for the report and to Tim Graham for the review.

docs/topics/forms/index.txt

@@ -341,6 +341,16 @@ from that ``{{ form }}`` by Django's template language.
     directly tied to forms in templates, this tag is omitted from the
     following examples in this document.
 
+.. admonition:: HTML5 input types and browser validation
+
+    If your form includes a :class:`~django.forms.URLField`, an
+    :class:`~django.forms.EmailField` or any integer field type, Django will
+    use the ``url``, ``email`` and ``number`` HTML5 input types. By default,
+    browsers may apply their own validation on these fields, which may be
+    stricter than Django's validation. If you would like to disable this
+    behavior, set the `novalidate` attribute on the ``form`` tag, or specify
+    a different widget on the field, like :class:`TextInput`.
+
 We now have a working web form, described by a Django :class:`Form`, processed
 by a view, and rendered as an HTML ``<form>``.