zfs(8): add requirements towards fs names

Provide explicit requirements towards file system naming convention
in OpenZFS man pages.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Mitigates #13310
Closes #13315