Resolve interaction between TH added documentation and user given documentation

Before !6224 (closed) the docs_decls was a Map Name HsDoc, it's now a Map Name [HsDoc], this raised the question about what to do when there is documentation added both by TH and by the user? Before we would prefer the documentation added by Haddock (see DocsInHiFileTH test) but perhaps we should combine it?

I think in order to preserve behaviour the correct thing to do is prefer docs added by TH but I open this ticket to raise the question in case someone asks.