Further haddocks improvements in Language.Haskell.Syntax.Pat.Pat

This is followup to !12794 (closed) As it turns out there were more outdated references to exactprint stuff. Also making the small syntax examples in haddocks more consistent.

CC @bgamari: this is what I asked you about in person on ZuriHac, whether it's preferable to just delete the outdated exactprint references in haddocks or to update them - you preferred updating so here it is. I'd like to do similar haddock improvements for other AST datatypes, so shout if you don't agree with this direction.