docs/170764: [zfs] 2399 zfs manual page does not document use of "zfs diff"

[araujo at FreeBSD.org]

Synopsis: [zfs] 2399 zfs manual page does not document use of "zfs diff"

Responsible-Changed-From-To: freebsd-doc->mm
Responsible-Changed-By: araujo
Responsible-Changed-When: Sun Aug 19 08:41:23 UTC 2012
Responsible-Changed-Why: 
Maybe Martin, could have time to take care of this PR.

http://www.freebsd.org/cgi/query-pr.cgi?pr=170764