Update info on referencing subseries

[ajeanmahoney]

While the referencegroup tag can still be used to build a subseries reference, there are now xi:include URLs for subseries references that can be used. Instead of:

<referencegroup anchor="STD78" target="https://www.rfc-editor.org/info/std78">
  <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5343.xml"/>
  <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5590.xml"/>
  <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5591.xml"/>
  <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6353.xml"/>
</referencegroup>

You can use:

<xi:include href="https://bib.ietf.org/public/rfc/bibxml9/reference.STD.78.xml"/>

To create a xref to an RFC within a subseries, use the following:

Section <xref target="RFC6353" section="1" sectionFormat="bare"/> of RFC 6353 <xref target="STD78"/>

[ajeanmahoney]

This info should go here: https://authors.ietf.org/en/references-in-rfcxml