fix: clarify help text

@marmoute , @martinvonz : I think you've both worked on this extension a fair amount in the past, so I want to make sure I'm not misunderstanding something before spending a bunch more time on it. Or if you can save me some time by pointing me in the right direction if the described behavior sounds familiar, that's helpful too. I didn't go back in time to see how long this has been the case, but it seems quite surprising that nobody else has hit it, so maybe it's a new-ish regression. I tried recreating the file not found error I hit the other day, unsuccessfully here. But I've got an old 6.2.1 on PATH, so maybe that was fixed since then.