Fix hints on help pages. Closes issue #141.

Update issue #141.
It looks like we're actually quite a bit better at figuring out
element box data than Firebug is. Aside from actually mispositioning
the layout box, Firebug actually chokes on our pages and can't
manage to remove the layout guides once it's inserted them. Score
one for our hint system, I guess.

common/content/hints.js

@@ -325,9 +325,10 @@ function Hints() //{{{
             pageHints.push([elem, text, span, null, elem.style.backgroundColor, elem.style.color, showText]);
         }
 
-        if (doc.body)
+        let body = doc.body || util.evaluateXPath(["body"], doc).snapshotItem(0);
+        if (body)
         {
-            doc.body.appendChild(fragment);
+            body.appendChild(fragment);
             docs.push({ doc: doc, start: start, end: pageHints.length - 1 });
         }