Update zfs(8) Snapshots section

The Snapshots section of the zfs(8) man page is incorrect and should
have been updated as part of #1312. Snapshots of volumes can be
accessed independently and their visibility is determined by the
'snapdev=hidden|visible' property. This is analogous to the existing
'snapdir=hidden|visible' property.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes #1921