Allow to override the docstrings for fields

[rixx]

If I have a simple model, and the verbose_name or help_text is public-facing and not what I'd like to see in my developer docs, I want to override the sphinxcontrib-django doc string by providing my own – but instead of overriding, I now have the same field documented twice.

(Bonus bug: the (SomeField) annotation provided by this package is found with the added to my additional documentation, not to the one generated by this package.)

[vdboor]

Merged, and fixed! Thanks!