The status of the documentation being hidden or exposed is due to the {-# OPTIONS_HADDOCK hide #-} Though, honestly, in my opinion we should still generate the documentation in this case, just not list it in the default view. Excerpts from Andrés Sicard-Ramírez's message of 2016-02-26 06:01:16 -0800:

Dear all,

By running in GHC 7.10.3

$ ghc-pkg field base-4.8.2.0 exposed-modules

I can see various modules exposed by base-4.8.2.0 which are not documented (e.g. GHC.NUm) in

http://downloads.haskell.org/~ghc/7.10.3/docs/html/libraries/index.html

Which is the criterion for adding GHC documentation to the modules exposed by base?

All the best,