doc: update rebalance doc, add SSL/TLS doc

Change-Id: I5d21aabaa6bfa6de40459c9a1969832a0a5d75e1
Signed-off-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-on: http://review.gluster.org/8961
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Xavier Hernandez <xhernandez@datalab.es>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>