[1.6.x] Added release note for TypedChoiceField coerce limitation

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

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

* Due to a change in the form validation workflow,

  :class:`~django.forms.TypedChoiceField` ``coerce`` method should always

  return a value present in the ``choices`` field attribute. That limitation

  should be lift again in Django 1.7.

* 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