[1.4.x] Fixed #14885 - Clarified that ModelForm cleaning may not fully...

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The first time you call ``is_valid()`` or access the ``errors`` attribute of a

``ModelForm`` triggers form validation as well as :ref:`model validation

<validating-objects>`. This has the side-effect of cleaning the model you pass

to the ``ModelForm`` constructor. For instance, calling ``is_valid()`` on your

form will convert any date fields on your model to actual date objects.

``ModelForm`` triggers :ref:`form validation <form-and-field-validation>` as

well as :ref:`model validation <validating-objects>`. This has the side-effect

of cleaning the model you pass to the ``ModelForm`` constructor. For instance,

calling ``is_valid()`` on your form will convert any date fields on your model

to actual date objects. If form validation fails, only some of the updates

may be applied. For this reason, you'll probably want to avoid reusing the

model instance.

The ``save()`` method