Clarified what a Quad is

This was suggested by @cygri, thanks.

This addresses #125.

spec/latest/json-ld-api/index.html

@@ -903,10 +903,10 @@ <h3>JsonLdOptions</h3>
 
     <section>
     <h3>Quad</h3>
-    <p>The <a>Quad</a> interface represents an RDF Quad.
-      See [[!RDF-CONCEPTS]] definition for
-      <cite><a href="http://www.w3.org/TR/rdf11-concepts/#dfn-rdf-triple">RDF triple</a></cite>,
-      which most closely aligns to <a>Quad</a>.
+    <p>The <a>Quad</a> interface represents an RDF Quad. An RDF Quad is an
+      <cite><a href="http://www.w3.org/TR/rdf11-concepts/#dfn-rdf-triple">RDF triple</a></cite>
+      [[!RDF-CONCEPTS]] with an optional fourth element, the graph name, being a
+      <ldtref>Node</ldtref>.
     </p>
     <dl title="[NoInterfaceObject] interface Quad" class="idl">
       <dt>readonly attribute Node subject</dt>