Clarify how profile parameter is used in content negotiation

... as suggested by Peter Ansell.

This closes RDF-ISSUE-128.

spec/latest/json-ld/index.html

@@ -160,6 +160,8 @@
     <li>Allow keyword aliases to be defined using expanded term definitions</li>
     <li>Mention profile URIs also in the sections describing the various document forms</li>
     <li>Support relative IRIs in <code>@base</code></li>
+    <li>Clarify how the <code>profile</code> media type parameter is used in the content
+      negotiation process</li>
   </ul>
 </section>
 
@@ -3644,8 +3646,10 @@ <h3>application/ld+json</h3>
             A profile does not change the semantics of the resource representation
             when processed without profile knowledge, so that clients both with
             and without knowledge of a profiled resource can safely use the same
-            representation. The <code>profile</code> parameter may be used by
+            representation. The <code>profile</code> parameter MAY be used by
             clients to express their preferences in the content negotiation process.
+            If the profile parameter is given, a server SHOULD return a document that
+            honors the profiles in the list which are recognized by the server.
             It is RECOMMENDED that profile URIs are dereferenceable and provide
             useful documentation at that URI. For more information and background
             please refer to [[RFC6906]].</p>