8435 zpool.1m and zfs.1m: minor cleanup

illumos/illumos-gate@a058d1cc571af5fbcfe7f1d719df1abbfdb722f3
https://github.com/illumos/illumos-gate/commit/a058d1cc571af5fbcfe7f1d719df1abbfdb722f3

https://www.illumos.org/issues/8435

This commit is a result of re-read while porting zpool.1m to ZFSonLinux.
See https://github.com/zfsonlinux/zfs/commit/
cda0317e4d2a1277b328e4fc42ee3699bbe46c12

https://www.illumos.org/issues/3796

The listsnapshots property is not documented in the zpool man page.

3796 listsnapshots not documented in zpool man page
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Yuri Pankov <yuripv@gmx.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: George Melikov <mail@gmelikov.ru>