doc: added documentation for dispersed volumes

Change-Id: I8a8368bdbe31af30a239aaf8cc478429e10c3f57
BUG: 1148089
Signed-off-by: Xavier Hernandez <xhernandez@datalab.es>
Reviewed-on: http://review.gluster.org/8893
Reviewed-by: Dan Lambright <dlambrig@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>