A common misconception is that one can create a backport by issuing `/backport` as a command on the PR, rather than the actual commit. I've lost count how many times someone ask why this does not work.

Ideally, I think this should be allowed, and interpreted as having entered `/backport` on the commit associated with the PR, but if this is too tricky to implement, we should at the very least add a response like:

"(!) You cannot issue `/backport` on a pull request. Instead, give that command on the actual commit you want to backport."

instead of an invalid command response.