Fix the description of oauth2_revoke_token
Fix the description of oauth2_revoke_token

Also tweak some of the other documentation in AuthClient to be a little
more conformant to the standard sections we have.

Closes #63

globus/globus-sdk-python,aaschaer/globus-sdk-python,sirosen/globus-sdk-python,globusonline/globus-sdk-python,globus/globus-sdk-python