docs: public static String describeExtensionContext(StructureDefinition ext) {
      CommaSeparatedStringBuilder b = new CommaSeparatedStringBuilder();
      for (StringType t : ext.getContext())
        b.append(t.getValue());
      if (!ext.hasContextType())
        throw new Error("no context type on "+ext.getUrl());
      switch (ext.getContextType()) {
      case DATATYPE: return "Use on data type: "+b.toString();
      case EXTENSION: return "Use on extension: "+b.toString();
      case RESOURCE: return "Use on element: "+b.toString();
      case MAPPING: return "Use where element has mapping: "+b.toString();
      default:
        return "??";
      }
    }