Documentation for GHC.IO.Exception

[Phil Ruffwind]

On Hackage, there seems to be no documentation for GHC.IO.Exception in
base, but the package can in fact be imported so it's not exactly an
internal package.

directory and likely many other packages do use the GHC-specific error
types like InappropriateType in exceptions, so it would be useful if
there were a documentation page for these things even if there isn't
any text.  As it is right now, the discoverability of these error
types is very low as you need to know the magical URL to show the
source code:

https://hackage.haskell.org/package/base/docs/src/GHC.IO.Exception.html

That's also partly a problem with Haddock; AFAIK there's no way to
navigate to the Source Code packages of modules whose documentation is
disabled, even though they are in fact present if you can figure out
the URL.

So would it be OK to open up this module or is there a reason for
keeping them discreet?