Try to clarify frame packing.

Marko was concerned that RFC 6716 section 3.2.1 narrowly describes
zero-length DTX frames _only_ for code 2 and 3 packets, and therefore
wanted this sentence to state clearly that code 0 and 1 can be used
with zero-byte frames as well. I've tried to do that.

doc/draft-ietf-codec-oggopus.xml

@@ -264,6 +264,16 @@ The actual length of each missing Opus frame inside the packet is zero bytes,
  as defined in Section&nbsp;3.2.1 of&nbsp;<xref target="RFC6716"/>.
 </t>
 
+<t>
+Zero-byte frames MAY be packed into packets using any of codes&nbsp;0, 1,
+ 2, or&nbsp;3.
+When successive frames have the same configuration, the higher code packings
+ reduce overhead.
+Likewise, if the TOC configuration matches, the muxer MAY further combine the
+ empty frames with previous or subsequent non-zero-length frames (using
+ code&nbsp;2 or VBR code&nbsp;3).
+</t>
+
 <t>
 <xref target="RFC6716"/> does not impose any requirements on the PLC, but this
  section outlines choices that are expected to have a positive influence on
@@ -325,13 +335,6 @@ Since medium-band audio is only supported in the SILK modes, wideband frames
  will be able to preserve all of the available audio bandwidth.
 </t>
 
-<t>
-Matching synthetic TOC byte(s) MAY be packed into packets using any of
- codes&nbsp;0, 1, 2, or&nbsp;3.
-If the TOC configuration matches, the muxer MAY further combine the empty
- frames with previous or subsequent non-zero-length frames (using code&nbsp;2
- or VBR code&nbsp;3).
-</t>
 </section>
 
 <section anchor="preskip" title="Pre-skip">