List substantive changes since Rec in Status section (#547)

This adjusts the Status of this Document section to list the two new features
that were added since publication as a Recommendation in 2017, note that
inclusion of additional features is out of scope, and list substantive changes
that were made to address maintenance issues.

Fixes #546.

encrypted-media-respec.html

@@ -95,7 +95,42 @@
         license or other server.
       </p>
     </section>
-    <section id="sotd"></section>
+    <section id="sotd">
+      <p>
+        Two new features were added since publication as a <abbr title=
+        "World Wide Web Consortium">W3C</abbr> Recommendation in <a href=
+        "https://www.w3.org/TR/2017/REC-encrypted-media-20170918/">September 2017</a>:
+      </p>
+      <ul>
+        <li>Encryption scheme capability detection, through the
+        {{MediaKeySystemMediaCapability/encryptionScheme}} attribute.
+        </li>
+        <li>The ability to query the status of a key associated with an HDCP policy, through the
+        {{MediaKeys/getStatusForPolicy()}} method.
+        </li>
+      </ul>
+      <p>
+        Inclusion of other features is out of scope for the Media Working Group. On top of
+        editorial updates, other substantive changes made to this specification address maintenance
+        issues against this specification:
+      </p>
+      <ul>
+        <li>Integrate with the Permissions Policy through the [=encrypted-media=] identifier.
+        </li>
+        <li>Add {{MediaKeyStatus/"usable-in-future"}} to {{MediaKeyStatus}} for keys that are not
+        yet usable for decryption.
+        </li>
+        <li>Return [=QuotaExceededError=] when steps fail due to a lack of resources.
+        </li>
+        <li>Add {{MediaKeySessionClosedReason}} to describe possible reasons for a session closure,
+        and adjust algorithms accordingly.
+        </li>
+      </ul>
+      <p>
+        For a full list of changes made since the previous version, see the <a href=
+        "https://github.com/w3c/encrypted-media/commits/main">commits</a>.
+      </p>
+    </section>
     <section class="informative">
       <h2>
         Introduction