doc: update details about posix acl on nfs mount

Change-Id: I591f2393a11893c77990a5b994196cb0a01358aa
BUG: 1086774
Signed-off-by: Humble Chirammal <hchiramm@redhat.com>
Reviewed-on: http://review.gluster.org/7871
Reviewed-by: Santosh Pradhan <spradhan@redhat.com>
Reviewed-by: Lalatendu Mohanty <lmohanty@redhat.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Niels de Vos <ndevos@redhat.com>