jail: unbreak the zfs.mount_snapshot option

The change of its description from integer to boolean didn't actually
change it to a boolean, but only made it impossible to set as either
a boolean or an integer.

Rather than make it work as a boolean parameter should, just revert
to the old (working) integer parameter, and change the documentation
to match.

PR: 274263
Reported by: andrew.hotlab at hotmail