Add documentation for disk parameters to man pages

Document the stripes parameter for the plain template and the
resync-rate, datastripes, metastripes, disk-barriers, meta-barriers,
metavg, disk-custom and meta-custom parameters for the drbd template.

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>