Add notes to ghc-prim Haddocks that users should not import it

A cheap way of achieving some of the ultimate goal of https://github.com/ghc-proposals/ghc-proposals/pull/528

I haven't checked the generated Haddock files because building them requires cabal, and cabal insists on building the source, and the source of ghc-prim can't be built because "only already installed instances can be used".