survey: Add comment

Author: nh2

survey/default.nix

@@ -432,6 +432,11 @@ let
         # directly to GHC.
         # Tip: If you want to debug this when it's failing, see
         #      https://github.com/NixOS/nixpkgs/issues/65210#issuecomment-513515829
+        # A common reason for it to fail is when the wrong `compiler` is given;
+        # in that case, the build log of the `Cabal` package involved will show
+        # two different ghc versions, and the output's `lib` directory will also
+        # contain 2 different ghc versions (one with the `.o` files and one with
+        # the `.conf` file).
         preCompileBuildDriver = ''
           cabalPackageId=$(basename --suffix=.conf ${fixedCabal}/lib/ghc-*/package.conf.d/*.conf)
           echo "Determined cabalPackageId as $cabalPackageId"