How to build Haddock documentation quickly?

[Norman Ramsey]

In service of #20528, I'm trying to clean up some Haddock
documentation in the GHC sources.  The only command I know is

  ./hadrian/build -j _build/docs/html/index.html

It turns out that if I change Haddock comments in one file
(GHC/Tc/Gen/Foreign.hs), rebuilding the HTML takes a minute
and a quarter.  That's a little long.  (And I had one build
take over four minutes, which is even longer.)

What tricks can I use to speed up this process?  Might there be a way
to run Haddock directly until things are to my liking?


Norman