doc: quota-scalability documentation

Change-Id: Ia6516293a7d6dc2643bb05767875b0cb651a65ef
BUG: 1086754
Signed-off-by: Susant Palai <spalai@redhat.com>
Reviewed-on: http://review.gluster.org/7882
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Tested-by: Niels de Vos <ndevos@redhat.com>