doc: afr-locks and its evolution

History and current state of how AFR uses the inode and entry locks
provided by the locks xlator

Thanks to Pranith for patiently explaining this.

Change-Id: I25d21c6361e95aa0b3edcffe9b3b9e470f488bc6
BUG: 1118309
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/8364
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>