Added refs to python documentation. Made minor language fixes.

This is a small modification to the documentation, replacing ``code`` by intersphinx refs to python documentation when it is justifiable.
I also made some minor tweaks on the English.

This is my first pull request, that is why is so minor. It is part of my way on learning on how to commit to Django.

I intend to continue to review the documentation until I feel confortable enough to change code.