Allow users to find "--" argument documentation

[Dan Jacobson]

If we read
(info "(coreutils) cat invocation")
we also get told about
(info "(coreutils) Common options")

But if we read e.g.,
(info "(find) Invoking xargs")
we never know about
(info "(coreutils) Common options")
even though they also apply.
So unlike cat, for xargs, even if the user has exhausted all the
documentation steps, man, --help, INFO, he will never learn about "--".

Sure, (info "(coreutils) Common options") says "In fact, every GNU
program accepts ... these options."
That's great, but some don't directly or indirectly document that fact.