Disallow combining Global with Constructor/NamedConstructor. (#745)

Fixes #744.

Author: Ms2ger

index.bs

@@ -8276,6 +8276,9 @@ The [{{Constructor}}] and
 [{{NoInterfaceObject}}]
 extended attributes must not be specified on the same interface.
 
+The [{{Constructor}}] and [{{Global}}] [=extended attributes=] must not be specified on the same
+[=interface=].
+
 See [[#interface-object]] for details on how a constructor
 for an interface is to be implemented.
 
@@ -9229,6 +9232,9 @@ that has an [=interface object=],
 and must not be one of the
 [=reserved identifiers=].
 
+The [{{NamedConstructor}}] and [{{Global}}] [=extended attributes=] must not be specified on the
+same [=interface=].
+
 See [[#named-constructors]] for details on how named constructors
 are to be implemented.