haskell.packages.ghc8107.wai-extra: disable haddock

Apparently, the haddock version shipped with GHC < 9.0 doesn't support
haddock syntax like this:

    module Foo
      -- * Heading
      ( bar
      -- …
      )

pkgs/development/haskell-modules/configuration-ghc-8.10.x.nix

@@ -119,6 +119,10 @@ self: super: {
     algebraic-graphs = dontCheck self.algebraic-graphs_0_6_1;
   });
 
+  # Uses haddock placement that isn't supported by the versions of haddock
+  # bundled with GHC < 9.0.
+  wai-extra = dontHaddock super.wai-extra;
+
   # Overly-strict bounds introducted by a revision in version 0.3.2.
   text-metrics = doJailbreak super.text-metrics;