Adding `-haddock` no longer makes the parser more strict

From https://downloads.haskell.org/ghc/9.6.1/docs/users_guide/using.html#haddock:

Note that this flag makes GHC’s parser more strict so programs which are accepted without Haddock may be rejected with -haddock.

I think this is no longer true after !2377 (closed), so this paragraph could either be deleted our replaced with a reference to -Winvalid-haddock.