Add realm note

Author: shaseley

dom.bs

@@ -1978,9 +1978,13 @@ an optional <var>reason</var>, run these steps:
 interface that inherits from it, and an optional <var>realm</var>, run these steps:
 
 <ol>
- <li><p>Let <var>resultSignal</var> be a <a for=/>new</a> object implementing
- <var>signalInterface</var> using <var>realm</var> if given, otherwise using the default behavior
- defined in Web IDL.
+ <li>
+  <p>Let <var>resultSignal</var> be a <a for=/>new</a> object implementing
+  <var>signalInterface</var> using <var>realm</var> if given, otherwise using the default behavior
+  defined in Web IDL.
+
+  <p class=XXX>As of the time of this writing Web IDL does not yet define any default behavior;
+  see <a href="https://github.com/whatwg/webidl/issues/135">whatwg/webidl#135</a>.
 
  <li><p>Set <var>resultSignal</var>'s [=AbortSignal/composite=] to true.