Clarify [[OwnPropertyKeys]] for named properties objects

See whatwg/html#9068.

Author: delan

index.bs

@@ -11430,6 +11430,12 @@ for that interface on which named properties are exposed.
     1.  Return |obj|.
 </div>
 
+Note: The \[[OwnPropertyKeys]] internal method of a named properties object
+continues to use <a abstract-op>OrdinaryOwnPropertyKeys</a>,
+unlike the counterpart for [=legacy platform objects=].
+Since named properties are not “real” own properties,
+they will not be returned by this internal method.
+
 The [=class string=] of a [=named properties object=]
 is the concatenation of the [=interface=]'s
 [=identifier=] and the string "<code>Properties</code>".