Revision 239394

Update zfs(8) manpage with illumos version of "zfs diff"

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

References:
  https://www.illumos.org/issues/2399

PR:		docs/170764
Obtained from:	ssh://anonhg@hg.illumos.org/illumos-gate
MFC after:	1 week