8605 zfs channel programs: zfs.exists undocumented and non-working

illumos/illumos-gate@5f39f884e2035d671ec02148fc4d8420c670bcb4
https://github.com/illumos/illumos-gate/commit/5f39f884e2035d671ec02148fc4d8420c670bcb4

https://www.illumos.org/issues/8605

zfs.exists() in channel programs doesn't return any result, and should have a
man page entry.

Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Chris Williamson <chris.williamson@delphix.com>