auth docs don't say they depend on contenttypes

[Kenneth Arnold]

The django.contrib.auth docs do not indicate that it depends on django.contrib.contenttypes.

[arien]

Mention that django.contrib.auth.models.Permission depends on django.contrib.contenttypes

[Luke Plant]

(In [11487]) Fixed #7376: auth docs don't say they depend on contenttypes.

Thanks arien