[1.10.x] Fixed #27039 -- Fixed empty data fallback to model field default in model forms.

Backport of 4bc6b93994 from master

docs/topics/forms/modelforms.txt

@@ -330,6 +330,21 @@ Note that if the form :ref:`hasn't been validated
 ``form.errors``. A ``ValueError`` will be raised if the data in the form
 doesn't validate -- i.e., if ``form.errors`` evaluates to ``True``.
 
+If an optional field doesn't appear in the form's data, the resulting model
+instance uses the model field :attr:`~django.db.models.Field.default`, if
+there is one, for that field. This behavior doesn't apply to fields that use
+:class:`~django.forms.CheckboxInput` (or any custom widget with
+``dont_use_model_field_default_for_empty_data=True``) since an unchecked
+checkbox doesn't appear in the data of an HTML form submission. Use a custom
+form field or widget if you're designing an API and want the default fallback
+for a ``BooleanField``.
+
+.. versionchanged:: 1.10.1
+
+    Older versions don't have the exception for
+    :class:`~django.forms.CheckboxInput` which means that unchecked checkboxes
+    receive a value of ``True`` if that's the model field default.
+
 This ``save()`` method accepts an optional ``commit`` keyword argument, which
 accepts either ``True`` or ``False``. If you call ``save()`` with
 ``commit=False``, then it will return an object that hasn't yet been saved to