Update @none definition.

Fixes #228.

Author: gkellogg

index.html

@@ -588,7 +588,7 @@ <h2>Syntax Tokens and Keywords</h2>
       <dt class="changed"><code>@nest</code></dt><dd class="changed">Used to define a <a>property</a> of a <a>node object</a> that groups together properties of that node, but is not an edge in the graph.</dd>
       <dt class="changed"><code>@none</code></dt><dd class="changed">Used as an index value
         in an <a>index map</a>, <a>id map</a>, <a>language map</a>, <a>type map</a>, or elsewhere where a <a>map</a> is
-        used to index into other values.</dd>
+        used to index into other values, when the indexed node does not have the feature being indexed.</dd>
       <dt class="changed"><code>@prefix</code></dt><dd class="changed">
         With the value <code>true</code>, allows this <a>term</a> to be used to construct a <a>compact IRI</a>
         when compacting.</dd>