template-haskell: rewrite TExp and Code documentation

Summary

https://hackage.haskell.org/package/template-haskell-2.19.0.0/docs/Language-Haskell-TH.html#t:TExp

mentions that

typed quotes, written as [|| ... ||] where ... is an expression; if that expression has type a, then the quotation has type Q (TExp a)

However, [|| ... ||] produces Code Q a, as expected https://downloads.haskell.org/ghc/latest/docs/users_guide/exts/template_haskell.html#syntax

Proposed improvements or changes

Re-write docs for TExp and Code for newer compilers.

Environment

GHC 925