doc: where best to add ~ 400 words to the manual?

I would like to add material to the manual, detailing the differences
between the XPath 1.0 language supported in PG, and the XQuery/XPath 2.0+
expected by the standard.

A good preview of what that would look like is the "related to version
of XPath" wiki section at [1].

That wiki section (minus the two inside-baseball paragraphs about
commit 3963574 and its reversion) comes to a bit under 400 words.

So that's roughly the amount of material I'm thinking to add.
Where should it go?

I have thought of:

1. A new sect2 near the top of the "functions-xml" sect1. This makes sense
because the material applies to all the functions below that involve XPath.
But that seems like the wrong place for 400 words of gory details.

2. A short remark at the top of functions-xml, linked to a footnote with
the gory details. But that would be a long footnote. The longest currently
is a footnote with 101 words in the start tutorial.

3. A sect2 at the very end of functions-xml, linked to from a short remark
at the top.

4. A new appendix, or a new conformance section in the features.sgml
appendix, linked to from a short remark at the top of functions-xml.

Or is there an even better idea I have not thought of?

-Chap


[1]
https://wiki.postgresql.org/wiki/PostgreSQL_vs_SQL/XML_Standards#Related_to_version_of_XPath