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).

<openzfs#7734> (Swap deadlock in 0.7.9).

Singular, not plural, for non-supported swapping to a file.

Pull-request:  openzfs#14756
Signed-off-by: Graham Perrin <grahamperrin@freebsd.org>

man/man8/zfs-create.8

@@ -234,14 +234,11 @@ if the volume is not sparse.
 Print verbose information about the created dataset.
 .El
 .El
-.Ss ZFS Volumes as Swap
-ZFS volumes may be used as swap devices.
-After creating the volume with the
-.Nm zfs Cm create Fl V
-enable the swap area using the
-.Xr swapon 8
-command.
-Swapping to files on ZFS filesystems is not supported.
+.Ss ZFS for Swap
+Swapping to a ZFS volume is prone to deadlock and not recommended.
+See OpenZFS FAQ.
+.Pp
+Swapping to a file on a ZFS filesystem is not supported.
 .
 .Sh EXAMPLES
 .\" These are, respectively, examples 1, 10 from zfs.8