nixos/make-options-doc: check for manual paths in options.json

since we no longer use the docbook path the check there will no longer
fire. add one to optionsJSON to not lose this functionality.

nixos/lib/make-options-doc/default.nix

@@ -143,6 +143,14 @@ in rec {
         $baseJSON $options \
         > $dst/options.json
 
+    if grep /nixpkgs/nixos/modules $dst/options.json; then
+      echo "The manual appears to depend on the location of Nixpkgs, which is bad"
+      echo "since this prevents sharing via the NixOS channel.  This is typically"
+      echo "caused by an option default that refers to a relative path (see above"
+      echo "for hints about the offending path)."
+      exit 1
+    fi
+
       brotli -9 < $dst/options.json > $dst/options.json.br
 
       mkdir -p $out/nix-support