doc: afr-statistics command  documentation

BUG: 1086748
Change-Id: I67ca9958e4e369b5902fc32d8d9863073da76c77
Signed-off-by: Venkatesh Somyajulu <vsomyaju@redhat.com>
Reviewed-on: http://review.gluster.org/7792
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Tested-by: Niels de Vos <ndevos@redhat.com>