Describe @none in the @graph @index case, and note potential issu…

…es on merging multiple unindexed or unnamed graphs.

spec/latest/json-ld/index.html

@@ -3149,6 +3149,81 @@ <h3>Named Graph Indexing</h3>
 
   <p>As with <a>index maps</a>, when used with <code>@graph</code>, a container may also
     include <code>@set</code> to ensure that key values are always contained in an array.</p>
+
+  <p class="changed">The special index <code>@none</code> is used for indexing
+    graphs which does not have an <code>@index</code> key, which is useful to maintain
+    a normalized representation. <span class="note">Note, however, that
+      compacting a document where multiple unidentified named graphs are
+      compacted using the <code>@none</code> index will result in the content
+      of those graphs being merged. To prevent this, give each graph a distinct
+      <code>@index</code> key.</span></p>
+
+  <pre class="example nohighlight" data-transform="updateExample"
+       title="Indexing languaged-tagged strings using @none for no language">
+  <pre class="example nohighlight" data-transform="updateExample"
+       title="Indexing graph data in JSON-LD">
+  <!--
+  {
+    "@context": {
+       "@version": 1.1,
+       "schema": "http://schema.org/",
+       "name": "schema:name",
+       "body": "schema:articleBody",
+       "words": "schema:wordCount",
+       "post": {
+         "@id": "schema:blogPost",
+         "@container": ["@graph", "@index"]
+       }
+    },
+    "@id": "http://example.com/",
+    "@type": "schema:Blog",
+    "name": "World Financial News",
+    "post": {
+       "en": {
+         "@id": "http://example.com/posts/1/en",
+         "body": "World commodities were up today with heavy trading of crude oil...",
+         "words": 1539
+       },
+       "@none": {
+         "@id": "http://example.com/posts/1/de",
+         "body": "Die Werte an Warenbörsen stiegen im Sog eines starken Handels von Rohöl...",
+         "words": 1204
+       }
+    }
+  }
+  -->
+  </pre>
+
+  <p>This expands to the following:</p>
+
+  <pre class="example nohighlight" data-transform="updateExample"
+       title="Referencing named graphs after expansion">
+  <!--
+  [{
+    "@id": "http://example.com/",
+    "@type": ["http://schema.org/Blog"],
+    "http://schema.org/blogPost": [{
+      "@graph": [{
+        "@id": "http://example.com/posts/1/de",
+        "http://schema.org/articleBody": [{
+          "@value": "Die Werte an Warenbörsen stiegen im Sog eines starken Handels von Rohöl..."
+        }],
+        "http://schema.org/wordCount": [{"@value": 1204}]
+      }]
+    }, {
+      "@graph": [{
+        "@id": "http://example.com/posts/1/en",
+        "http://schema.org/articleBody": [{
+          "@value": "World commodities were up today with heavy trading of crude oil..."
+        }],
+        "http://schema.org/wordCount": [{"@value": 1539}]
+      }],
+      "@index": "en"
+    }],
+    "http://schema.org/name": [{"@value": "World Financial News"}]
+  }]
+  -->
+  </pre>
 </section>
 
 <section class="informative changed">
@@ -3552,7 +3627,11 @@ <h3>Named Graph Indexing by Identifier</h3>
   <p>As with <a>id maps</a>, the special index <code>@none</code> is used for indexing
     <a>named graphs</a> which do not have an <code>@id</code>, which is useful to maintain
     a normalized representation. The <code>@none</code> index may also be
-    a term which expands to <code>@none</code>.</p>
+    a term which expands to <code>@none</code>.
+    <span class="note">Note, however, that if multiple graphs are represented without
+      an <code>@id</code>, they will be merged on expansion. To prevent this,
+      use <code>@none</code> judiciously, and consider giving graphs
+      their own distinct identifier.</span></p>
 
   <pre class="example nohighlight" data-transform="updateExample"
        title="Referencing named graphs using an id map with @none">