[selectors] Add issue wrt more scoping types

Author: tabatkins

selectors/Overview.html

@@ -1374,6 +1374,10 @@ <h3 id=scoping><span class=secno>3.3. </span> Scoped Selectors</h3>
     but does not limit which elements match.
     <p> Scope-relative selectors must be <a href=absolutize>absolutized</a>
      before using them for matching.
+
+    <p class=issue> It might be necessary (for, e.g.
+     <code>::distributed()</code> or <code>documentFragment.find()</code>),
+     to split the concept of scope-relative selector into multiple concepts.
   </dl>
 
   <div class=example>

selectors/Overview.src.html

@@ -676,6 +676,10 @@ <h3 id="scoping">
 
       <p>
         Scope-relative selectors must be <a href="absolutize">absolutized</a> before using them for matching.
+
+      <p class="issue">
+        It might be necessary (for, e.g. <code>::distributed()</code> or <code>documentFragment.find()</code>),
+        to split the concept of scope-relative selector into multiple concepts.
   </dl>
 
   <div class='example'>