Fixed #21792 -- Documented Form.has_changed()

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Checking if form data has changed

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.. method:: Form.has_changed()

Use the ``has_changed()`` method on your ``Form`` when you need to check if the

form data has been changed from the initial data.

    >>> data = {'subject': 'hello',

    ...         'message': 'Hi there',

    ...         'sender': 'foo@example.com',

    ...         'cc_myself': True}

    >>> f = ContactForm(initial=data)

    >>> f.has_changed()

    False

When the form is submitted, we reconstruct it and provide the original data

so that the comparison can be done:

    >>> f = ContactForm(request.POST, initial=data)

    >>> f.has_changed()

``has_changed()`` will be ``True`` if the data from ``request.POST`` differs

from what was provided in :attr:`~Form.initial` or ``False`` otherwise.

Accessing the fields from the form

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