Clarify documentation of zfs destroy on snapshots (#17021)

The current documentation of zfs destroy in application to snapshots
is particularly difficult to understand. The following changes are made:

• Remove circular reference to zfs destroy in the documentation of that command.
• Remove use of "for example", which implies there are more, undocumented reasons that ZFS may fail to destroy a snapshot immediately.
• Mention properties defer_destroy and userrefs.
• Add zfsprops(8) to "SEE ALSO" list.
• Clarify meaning of -d option.

Requires-builders: none

Signed-off-by: mnrx <83848843+mnrx@users.noreply.github.com>
Co-authored-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: George Amanakis <gamanakis@gmail.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>