Improve Connection docs for use with Keystone v3

[Brian Curtin]

There are two separate but related improvements that should be made here:

1. Code examples, such as in doc/source/users/guides/connect.rst or in README.rst that shows up in PyPI, should be using Keystone v3. A lot of people get tripped up trying to auth to v3 without arguments like project_domain_id.

2. The Connection API docs should better explain that the arguments it takes are passed off to KSA and then link directly to said arguments. For example, password auth seems to be the most commonly used, and the true source of those arguments are at https://docs.openstack.org/keystoneauth/latest/api/keystoneauth1.identity.v3.html#keystoneauth1.identity.v3.PasswordMethod —we should find a good way to link straight to that and the other auth methods.