cli: Made volume top help string clear

nfs option is not applicable for read-perf and
write-perf
nfs option and brick option can not be used in
same command

Change-Id: I920ba0de011df0cc5e0adca6597aaea9372fe592
BUG: 924335
Signed-off-by: M S Vishwanath Bhat <vbhat@redhat.com>
Reviewed-on: http://review.gluster.org/4706
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-by: Raghavendra Talur <rtalur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>