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).