Fix typo and add comment

Fix spelling of "privileges" in gswauth-* commands.
Add comment to clarify use of req.credentials_valid.

Change-Id: Ibb4d331f57df29e28670252b1360b3a72ea688cf
Signed-off-by: Prashanth Pai <ppai@redhat.com>
Reviewed-on: http://review.gluster.org/6448
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>