Correct Session docstring
Correct Session docstring

The online docs for the Session class are unusable because the
arguments to __init__ were not displayed.

This and other issues in the docstrings for the class are corrected.

Change-Id: Ia03b785f132f7d21ba576c0c8d634051d7127319

citrix-openstack-build/keystoneauth,sileht/keystoneauth,jamielennox/keystoneauth