Improve documentation of django.contrib.redirects

[Adam Johnson]

I think there are a couple small improvements that could be made to the docs here:

• The "Via the Python API" section could do with a simple shell example for creating a Redirect, since otherwise the fields aren't mentioned in the docs. I also imagine it might be used by relatively new users who aren't confident with the ORM.
• The Redirect.new_path field's help_text states full URL starting with “http://””. This doesn't mention the now-more-common https://, nor the fact that arbitrary protocols are technically supported, such as onion://. Perhaps `full URL starting with a scheme such as "​https://