[1.6.x] Fixed #21339 -- Documented removal of some form field error messages

  changes in 1.6 particularly affect :class:`~django.forms.DecimalField` and

  :class:`~django.forms.ModelMultipleChoiceField`.

* Some :attr:`~django.forms.Field.error_messages` for

  :class:`~django.forms.IntegerField`,

  :class:`~django.forms.EmailField`,

  :class:`~django.forms.IPAddressField`,

  :class:`~django.forms.GenericIPAddressField`, and

  :class:`~django.forms.SlugField` have been suppressed because they

  duplicated error messages already provided by validators tied to the fields.

* There have been changes in the way timeouts are handled in cache backends.

  Explicitly passing in ``timeout=None`` no longer results in using the

  default timeout. It will now set a non-expiring timeout. Passing 0 into the