zfs-create(8): ZFS for swap: caution, clarity

zfs-create(8): ZFS for swap: caution, clarity

Make the section heading more generic (the section relates to ZFS files
as well as ZFS volumes).

Swapping to a ZFS volume is prone to deadlock. Remove the related
instruction, direct readers to OpenZFS FAQ. Related, but not linked
from within the manual page:

<https://openzfs.github.io/openzfs-docs/Project%20and%20Community/FAQ.html#using-a-zvol-for-a-swap-device-on-linux>
(Using a zvol for a swap device on Linux).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Graham Perrin <grahamperrin@freebsd.org>
Issue #7734
Closes #14756