Document low-level credentials methods
Document low-level credentials methods

This commit fills in missing documentation for low-level
credentials methods.

Closes #9

pythongssapi/python-gssapi,pythongssapi/python-gssapi,frozencemetery/python-gssapi,frozencemetery/python-gssapi,pythongssapi/python-gssapi,frozencemetery/python-gssapi