Improve normative definition of **JSON-LD document**, **node object**, and **graph object**.

Fixes #543.

Author: gkellogg

spec/latest/common/terms.html

@@ -127,7 +127,7 @@
     A <a>graph object</a> represents a <a>named graph</a> represented as the
     value of a <a>property</a> within a <a>node object</a>. When expanded, a
     graph object MUST have an <code>@graph</code> member, and may also have
-    <code>@context</code>, <code>@id</code>, and <code>@index</code> members.
+    <code>@id</code>, and <code>@index</code> members.
     A <dfn class="preserve" data-lt="simple graph objects">simple graph object</dfn> is a
     <a>graph object</a> which does not have an <code>@id</code> member. Note
     that <a>node objects</a> may have a <code>@graph</code> member, but are

spec/latest/json-ld/index.html

@@ -4370,9 +4370,10 @@ <h1>JSON-LD Grammar</h1>
       in the <a>JSON-LD internal representation</a> that is equivalent to
       valid <a data-cite="RFC7159#section-2">JSON text</a></span>.</p>
 
-  <p>A <a>JSON-LD document</a> MUST be a single <a>node object</a>
-    or an <a>array</a> whose elements are each
-    <a>node objects</a> at the top level.</p>
+  <p>A <a>JSON-LD document</a> MUST be a single <a>node object</a>,
+    a <a>JSON object</a> consisting of only
+    the members <code>@context</code> and/or <code>@graph</code>,
+    or an <a>array</a> or zero or more <a>node objects</a>.</p>
 
   <p>In contrast to JSON, in JSON-LD the keys in <a data-lt="JSON object">objects</a>
     MUST be unique.</p>
@@ -4425,7 +4426,8 @@ <h2>Node Objects</h2>
       <a>context</a> and:</p>
 
     <ul>
-      <li>it is the top-most <a>JSON object</a> in the JSON-LD document,</li>
+      <li>it is not the top-most <a>JSON object</a> in the JSON-LD document consisting
+        of no other members than <code>@graph</code> and <code>@context</code>,</li>
       <li>it does not contain the <code>@value</code>, <code>@list</code>,
         or <code>@set</code> keywords, and</li>
       <li class="changed">it is not a <a>graph object</a>.</li>
@@ -5124,7 +5126,6 @@ <h4>Open Issues</h4>
   <p class="issue defer" data-number="397"></p>
   <p class="issue defer" data-number="443"></p>
   <p class="issue defer" data-number="491"></p>
-  <p class="issue" data-number="543"></p>
   <p class="issue defer" data-number="547"></p>
   <p class="issue defer" data-number="548"></p>
   <p class="issue defer" data-number="583"></p>