Make classes without base class explicit

Author: Bolpat

spec/expression.dd

@@ -1674,7 +1674,10 @@ $(H3 $(LNAME2 super, super))
     $(P $(D super) is identical to $(D this), except that it is
         cast to $(D this)'s base class.
         It is an error if there is no base class.
-        It is an error to use the $(D super) reference outside a non-static class member function.
+        It is an error to use the $(D super) reference outside a non-static class member function,
+        or inside a member function of a class without a base class.
+        (The only `extern(D)` class without a base class is `Object`,
+        however, note that `extern(C++)` classes have no base class unless specified.)
         If a member function is called with an explicit reference
         to $(D super), a non-virtual call is made.
     )