zfsprops.7: Add note about comma-separation

This change primarily seeks to make implicit documentation explicit, as
it is not outright stated that options should be comma-separated, nor is
there a reason given for it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Daniel Ebdrup Jensen <debdrup@FreeBSD.org>
Closes #12579