Use docURL as start_url (#991)

Author: marcoscaceres

index.html

@@ -767,7 +767,8 @@ <h3>
           <li>If |json|["start_url"] doesn't [=map/exist=] or
           |json|["start_url"] is not a [=string=], return.
           </li>
-          <li>If |json|["start_url"] is the empty string, return.
+          <li>If the type of |json|["start_url"] is not [=string=], or if
+          |json|["start_url"] is the empty string, return.
           </li>
           <li>Let |start URL:URL| be the result of [=URL Parser|parsing=]
           |json|["start_url"], using |manifest URL| as the base URL.