This flag undoes the effect of a previous "-haddock" flag.  Having both flags makes it easier
for build systems to enable Haddock parsing in a set of global flags, but then disable it locally for
specific targets (e.g., third-party packages whose comments don't pass the validation in the latest GHC).

I added the flag to expected-undocumented-flags.txt since `-haddock` was alreadyin that list.