rc.conf(5): Add _limits, _login_class, and _oomprotect

Add a few very useful variables that might easily be overlooked, since
they're only documented in rc.subr(8) which might not be the first place
that people look.

At least _oomprotect has existed since 11.0-RELEASE, and doesn't appear
to be very well-known. While the others aren't as new, in my estimation,
a lot more people would use them if they knew about them.

While here, also add a reference to rc.subr(8) and login.conf(5), and
sort the variables alphabetically.

Reported by: Daniel Dettlaff <dmilith at gmail.com>
Reviewed by: ceri, gbe, 0mp, ygy, a.wolk, pauamma