nfs: update nfs.addr-namelookup description

replaced rpc-auth.addr.* with nfs.rpc-auth-* in
keeping with the latest option exposed to the user.

Change-Id: I418354a4d171dae28dd0befe8ba9e5ee893cd013
BUG: 965025
Signed-off-by: Rajesh Amaravathi <rajesh@redhat.com>
Reviewed-on: http://review.gluster.org/5049
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>