improve documentation of :doc and -haddock

I was preaching about :doc but people said "what? does not work here".

Of course it works for me since I have ghc option -haddock in my global cabal config since forever.

ghci does the right thing (it prints Try re-compiling with '-haddock'.) but GHC documantation feels incomplete:

• documentation for -haddock says what is happening (parse Haddock comments and include them in the interface file) but does not say why I should want this https://downloads.haskell.org/ghc/latest/docs/users_guide/using.html#ghc-flag--haddock

• documentation for :doc does not say that it needs these interface files, and sounds vague and dated ("experimental, might change with GHC 8.8") https://downloads.haskell.org/ghc/latest/docs/users_guide/ghci.html#ghci-cmd-:doc

(for context, https://club.tidalcycles.org/t/where-are-n-and-notes-documented/5109/8 )

... Can we just make -haddock the default in ghc? (I am not sure about implications - interface size and compile time might go up). Global -haddock was a bit risky before GHC version 9 (?) since it would occasionally refuse to generate code when a haddock comment was malformed. But I've never seen such an error recently.