[1.10.x] Refs #27186 -- Fixed model form default fallback for CheckboxSelectMultiple.

Backport of 87c5e7efeb from master

docs/ref/forms/widgets.txt

@@ -281,10 +281,11 @@ foundation for custom widgets.
         The method's result affects whether or not a field in a model form
         :ref:`falls back to its default <topics-modelform-save>`.
 
-        A special case is :class:`~django.forms.CheckboxInput`, which always
-        returns ``False`` because an unchecked checkbox doesn't appear in the
-        data of an HTML form submission, so it's unknown whether or not the
-        user actually submitted a value.
+        Special cases are :class:`~django.forms.CheckboxInput` and
+        :class:`~django.forms.CheckboxSelectMultiple`, which always return
+        ``False`` because an unchecked checkbox doesn't appear in the data of
+        an HTML form submission, so it's unknown whether or not the user
+        actually submitted a value.
 
 ``MultiWidget``
 ---------------

docs/releases/1.10.2.txt

@@ -18,10 +18,10 @@ Bugfixes
 * Disabled system check for URL patterns beginning with a '/' when
   ``APPEND_SLASH=False`` (:ticket:`27238`).
 
-* Fixed model form ``default`` fallback for ``MultiWidget``, ``FileInput``,
-  ``SplitDateTimeWidget``, ``SelectDateWidget``, and ``SplitArrayWidget``
-  (:ticket:`27186`). Custom widgets affected by this issue may need to
-  implement a :meth:`~django.forms.Widget.value_omitted_from_data` method.
+* Fixed model form ``default`` fallback for ``CheckboxSelectMultiple``,
+  ``MultiWidget``, ``FileInput``, ``SplitDateTimeWidget``, ``SelectDateWidget``,
+  and ``SplitArrayWidget`` (:ticket:`27186`). Custom widgets affected by this
+  issue should implement :meth:`~django.forms.Widget.value_omitted_from_data`.
 
 * Fixed a crash in ``runserver`` logging during a "Broken pipe" error
   (:ticket:`27271`).

docs/topics/forms/modelforms.txt

@@ -335,7 +335,8 @@ 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 whose
+:class:`~django.forms.CheckboxInput` and
+:class:`~django.forms.CheckboxSelectMultiple` (or any custom widget whose
 :meth:`~django.forms.Widget.value_omitted_from_data` method always returns
 ``False``) 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