document customizing authentication views (e.g. login)

[Chris Jerdonek]

The ​Setting test cookies section of the Django documentation illustrates checking cookies using a sample login() view function.

However, the ​Customizing authentication in Django section of the Django documentation doesn't provide any hints as to whether the out-of-the-box login view can be extended (e.g. to provide the functionality described in the example above).

I think it would be good to say something in the latter section about how one can go about extending or customizing views like the login view, or alternatively, saying that one must copy-and-paste (or fork) to customize it if extending isn't possible. The "Setting test cookies" section can then link to this new section. I think it's natural to add a section like this, especially because it's a natural question to ask after reading the example test-cookie code.

[Tim Graham]

I think this could be rolled into #21292, I've made a note of this ticket there.