Improve template-haskell haddocks
This MR adds Haddock comments for many of the declarations in Language.Haskell.TH.Lib.Syntax
which were previously missing them, as described by #15822 (closed). I primarily aimed for coverage; potential further improvements could include:
- Consistency in the notation used to describe the correlation between concrete syntax and the TH ASTs.
- Dealing with the entries that users should not touch, but are nonetheless exported, e.g.
Language.Haskell.TH.Lib.Syntax.counter
. - Organisation via an explicit export list.