mac(4): also list jails in the example enumeration of objects

[kevans]

Summary

The prison may also be considered part of the subject by way of its
ucred association, but I don't think this is significantly different
enough today than before recent work -- policies could have always
taken them into account, and some did (e.g., mac_bsdextended).

[ziaee]

Thanks for doing this!

[olce]

Oh, you're right, jails are more of an object than a subject.

Thanks.