Illumos #2399: zfs manual page does not document use of "zfs diff".

illumos/illumos-gate@3b8be6bf4fd2c744dfb8b5ce2a6c85ad0a2c8f75
Illumos changeset: 13773:00c2a08cf1bb

2399 zfs manual page does not document use of "zfs diff"

Reviewed by: Joshua M. Clulow <josh@sysmgr.org>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Dan McDonald <danmcd@nexenta.com>
Approved by: Robert Mustacchi <rm@joyent.com>

Ported-by: Etienne Dechamps <etienne.dechamps@ovh.net>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #940