Fixed #23602 -- Add comment on get_absolute_url regarding user input

defined. If it makes sense for your model's instances to each have a unique

URL, you should define ``get_absolute_url()``.

.. warning::

    You should avoid building the URL from un-validated user input, in order to

    reduce possibilities of link or redirect poisoning::

        def get_absolute_url(self):

            return '/%s/' % self.name

    If ``self.name`` is ``'/example.com'`` this returns ``'//example.com/'``

    which, in turn, is a valid schema relative URL but not the expected

    ``'/%2Fexample.com/'``.

It's good practice to use ``get_absolute_url()`` in templates, instead of

hard-coding your objects' URLs. For example, this template code is bad: