git_cl.py uses auth.py to authenticate to cr-buildbucket,
but when it does so, it caches the token under a key for the
current codereview host, not the buildbucket hostname.

This is confusing, but is the desired behavior, so that users
can use different credentials (e.g. chromium and google) when
communicating with buildbucket about CLs on different gerrit
hosts (e.g. chromium and chrome-internal).

To prevent other people from getting confused like I did,
add some documentation to this effect.

Change-Id: If16896d15423bbdecf9624393773ac01d0d16a66
Reviewed-on: https://chromium-review.googlesource.com/990674
Commit-Queue: Aaron Gable <agable@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Nodir Turakulov <nodir@chromium.org>