oggopus: Add Opus RTP as an informative reference.

We mention this in the description of Channel Mapping Family 0.
Might as well link to RFC 7587.

Review comment from Mo Zanaty.

doc/draft-ietf-codec-oggopus.xml

@@ -8,6 +8,7 @@
 <!ENTITY rfc6381 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.6381.xml'>
 <!ENTITY rfc6716 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.6716.xml'>
 <!ENTITY rfc6982 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.6982.xml'>
+<!ENTITY rfc7587 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.7587.xml'>
 ]>
 <?rfc toc="yes" symrefs="yes" ?>
 
@@ -851,6 +852,7 @@ There are three defined mapping families in this specification.
 <t>
 Allowed numbers of channels: 1 or 2.
 RTP mapping.
+This is the same channel interpretation as <xref target="RFC7587"/>.
 </t>
 <t>
 <list style="symbols">
@@ -1566,6 +1568,7 @@ The authors agree to grant third parties the irrevocable right to copy, use,
 <!--?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.3550.xml"?-->
  &rfc4732;
  &rfc6982;
+ &rfc7587;
 
 <reference anchor="flac"
  target="https://xiph.org/flac/format.html">