cluster/afr: change message description in afr-messages.h

Changed the description of AFR_MSG_SUBVOL_UP to make it more meaningful.

Change-Id: I30fa13c2e9a280a22d48e777d259d04a3b71deef
BUG: 1075611
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/8149
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Tested-by: Pranith Kumar Karampuri <pkarampu@redhat.com>