Clarify that index maps for named graphs do not name the graphs, but simply index them.

Author: gkellogg

index.html

@@ -10156,8 +10156,9 @@ <h2>Index Maps</h2>
       set to an array containing both <code>@graph</code> and
       <code>@index</code>, and optionally including <code>@set</code>. The
       value consists of the <a>node objects</a> contained within the <a>named
-      graph</a> which is named using the referencing key, which can be
-      represented as a <a>simple graph object</a>.</p>
+      graph</a> which is indexed using the referencing key, which can be
+      represented as a <a>simple graph object</a> if the value does
+      not include <code>@id</code>, or a <a>named graph</a> if it includes <code>@id</code>.</p>
   </section>
 
   <section class="changed">