Add note w.r.t. matchAll changes

tc39/ecma262#1716

spec.html

@@ -87,6 +87,7 @@ <h1>String.prototype.replaceAll ( _searchValue_, _replaceValue_ )</h1>
   </emu-alg>
 </emu-clause>
 
+<!-- Note: the changes to String.prototype.matchAll have already been merged upstream here: https://github.com/tc39/ecma262/pull/1716 -->
 <emu-clause id="sec-string.prototype.matchall">
   <h1>String.prototype.matchAll ( _regexp_ )</h1>
   <p>Performs a regular expression match of the String representing the *this* value against _regexp_ and returns an iterator. Each iteration result's value is an Array object containing the results of the match, or *null* if the String did not match.</p>