code: public ConstructorDocImpl getConstructorDoc(MethodSymbol meth) {
          assert meth.isConstructor() : "expecting a constructor symbol";
          ConstructorDocImpl result = (ConstructorDocImpl)methodMap.get(meth);
          if (result != null) return result;
          result = new ConstructorDocImpl(this, meth);
          methodMap.put(meth, result);
          return result;
      }
docs: Return the ConstructorDoc for a MethodSymbol.
  Should be called only on symbols representing constructors.