Recommend compression=on in zfs(8) dedup section

Recommend compression=on in zfs(8) dedup section

compression=lz4 depends on the lz4 feature being enabled, while
compression=on will let ZFS use either lzjb or lz4 where appropriate.
It also allows the documentation to not go out of date if/when ZFS
picks a new default in the future.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mike Swanson <mikeonthecomputer@gmail.com>
Closes #6614