[3.0.x] Corrected outputting BooleanField as HTML in forms docs.

Backport of 34a69c2 from master

docs/ref/forms/api.txt

@@ -470,10 +470,10 @@ Notice the following:
 
 * Each field type has a default HTML representation. ``CharField`` is
   represented by an ``<input type="text">`` and ``EmailField`` by an
-  ``<input type="email">``.
-  ``BooleanField`` is represented by an ``<input type="checkbox">``. Note
-  these are merely sensible defaults; you can specify which HTML to use for
-  a given field by using widgets, which we'll explain shortly.
+  ``<input type="email">``. ``BooleanField(null=False)`` is represented by an
+  ``<input type="checkbox">``. Note these are merely sensible defaults; you can
+  specify which HTML to use for a given field by using widgets, which we'll
+  explain shortly.
 
 * The HTML ``name`` for each tag is taken directly from its attribute name
   in the ``ContactForm`` class.