Clarify guidance on sending bugs upstream

Sometimes it's best for package maintainers to act as a go-between for users when forwarding bugs upstream, but not always. For example, if a bug is likely to involve discussion with upstream - perhaps because upstream will need more information in order to diagnose it, or perhaps because it's a feature request that the package maintainer doesn't feel strongly enough about to champion - then it can be more useful for a package maintainer to check that the bug isn't Debian-specific and then ask users to file the bug upstream themselves.

I've had users refuse to file bugs upstream even when asked, because the letter of this page seems to tell them not to. Since I don't believe that the intent of the original change (see https://bugs.debian.org/189157) was to be quite as prescriptive as that, make it clear that package maintainers might reasonably ask for this.

(Of course, sometimes users won't want to create an account on upstream's bug tracker, or whatever. But that doesn't mean package maintainers can't ask.)