No expected undocumented flags

The GHC repo contains this file of shame (credit to @monoidal for telling me about it): https://gitlab.haskell.org/ghc/ghc/-/blob/master/docs/users_guide/expected-undocumented-flags.txt

Some dedicated attention is all that it would take to fix this. Any volunteers?

This would also fix #18630 (closed) and #18631 (closed) along the way. #18632 mentions a language extension that is missing from that file (possibly because it starts with No and the grepping command that produced the file probably filtered out -XNo.... extensions). #14985, #9358, and #9248 (closed) are also in this space.