Forbid overloading operations with Promise and non-Promise return types

Fixes the important part of https://www.w3.org/Bugs/Public/show_bug.cgi?id=25051. See #879 for a followup on general issues with the "return type" concept.

Author: Ms2ger

index.bs

@@ -3329,6 +3329,11 @@ that overloaded operations and constructors can be
 specified to take, and in order to describe these restrictions,
 the notion of an <em>effective overload set</em> is used.
 
+A set of overloaded [=operations=] must either:
+
+* contain no [=operations=] whose [=return type=] is a [=promise type=];
+* only contain [=operations=] whose [=return type=] is a [=promise type=].
+
 [=Operations=] must not be overloaded across
 [=interface=],
 [=partial interface=],