Update controller dfn, towards #3 and towards #4

terms.html

@@ -79,8 +79,11 @@
   <dt><dfn data-lt="did controller(s)">DID controller</dfn></dt>
 
   <dd>
-The entity, or a group of entities, in control of a DID and/or <a>DID document</a>.
-Note that the <a>DID controller</a> might include the <a>DID subject</a>.
+The entity that has the ability to make changes to a <a>DID document</a>. 
+A <a>DID</a> can have more than one controller. The <a>DID controller(s)</a>
+are denoted by the `controller` property on the top level of the <a>DID
+document</a>. Note that the <a>DID controller(s)</a> might include the
+<a>DID subject</a>.
   </dd>