librbdpy: Added missing method docstrings

Several methods were missing docstrings, preventing the methods
from appearing in the generated documentation.  Ensured all methods
now have appropriate docstrings.

Fixes: 5977
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
Merge pull request #2720 from ceph/wip-5977

librbdpy: Added missing method docstrings

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>