Mark blank nodes for properties and graph names at risk

... as discussed in today's RDF WG telecon.

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

@@ -2566,6 +2566,10 @@ <h1>Data Model</h1>
       <tref title="JSON-LD value">JSON-LD values</tref>.</li>
   </ul>
 
+  <p class="issue">In contrast to the RDF data model as defined in [[RDF-CONCEPTS]], JSON-LD allows
+    blank nodes as property labels and graph names to enable a number of use cases. This feature is
+    discussed controversially in the RDF WG and may be removed in the future.</p>
+
   <p><tref title="JSON-LD document">JSON-LD documents</tref> MAY contain data that cannot be
     represented by the <tref title="JSON-LD data model">data model</tref> defined above.
     Unless otherwise specified, such data is ignored when a <tref>JSON-LD document</tref>