[GHC-17916] isn't documented on `errors.haskell.org`

Which I discovered when someone unexpectedly got it under ghc 9.8.2 (shouldn't it only be an error in ghc 9.12+?).

It appears this was removed in 6acaff2b8094d20ff65d459dcc086f20fe0c0bc1, but as compilers in the wild still issue it, it might be a good idea to document it perhaps with a link to [GHC-78249].