Detect invalid Haddocks

Author: mrkkrp

mmark-cli.cabal

@@ -48,7 +48,7 @@ executable mmark
     if flag(dev)
         ghc-options:
             -Wall -Werror -Wredundant-constraints -Wpartial-fields
-            -Wunused-packages
+            -Wunused-packages -haddock -Winvalid-haddock
 
     else
         ghc-options: -O2 -Wall