for getElementById, clarify result when multiple elements have the sa…

…me id. Related to #182.

master/struct.html

@@ -2545,6 +2545,9 @@ <h3 id="InterfaceSVGSVGElement">Interface SVGSVGElement</h3>
 <p>The <b id="__svg__SVGSVGElement__getElementById">getElementById</b> method
 searches the descendant elements of the <a>'svg'</a> element that this method is
 called on for an element with an <a>'id'</a> attribute equal to <var>elementId</var>.
+Which element must be returned, when more than one element within the search
+space has an <a>'id'</a> attribute equal to <var>elementId</var>, is not
+defined.
 This method is deprecated and kept only for compatibility with legacy content.
 Authors are encouraged to use alternative methods.</p>
 <p>When getElementById(<var>elementId</var>) is called, the following steps