Clarify more things about what works inside `qt{}` strings

[leonerd]

The original text did not explicitly mention that regular $foo interpolation does not work here, but it seemed implied so I've added some words.

I've also added a guess at some words to try to clarify what to do with \x-style escapes. I think people would get upset if they couldn't at least have \n for a linefeed, so by extension I suspect any of the escapes that just stand in for some other literal character would be fine. The only problem ones are the case/quotmeta ones, so I've explicitly disallowed those.