Add note on restrictions on the use of property-based data indexing.

Author: gkellogg

index.html

@@ -6755,6 +6755,11 @@ <h3>Using the Document Base for the Default Vocabulary</h3>
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     </pre>
   </aside>
+
+  <p class="note">When using property-based data indexing, <a>index maps</a> can only be used
+    on <a>node objects</a>, not <a>value objects</a> or <a>graph objects</a>.
+    <a>Value objects</a> are restricted to have only certain keys and do not support
+    arbitrary properties.</p>
 </section>
 
 </section>