Improve documentation of netrc authentication

The documentation does not make it clear that when the credentials
from netrc are used, Requests authenticates with HTTP Basic Auth.

I just spent ages trying to figure out why it wasn't working, and
it was because although the credentials in the netrc were correct,
the server actually required HTTP Digest Auth.

Add a section in the authentication documentation to make it clear
that HTTP Basic Auth is used.
Move netrc authentication documentation under the Basic Auth section
Merge pull request #1465 from dpursehouse/doc-netrc-authentication

Improve documentation of netrc authentication