Possibly allow tei:note within tei:ref for footnotes

[wlpotter]

Sparked by #1936. We've run into cases where we need to add footnotes (<note type="footnote">) within a tei:ref element. We'll want to review these cases to see if it's possible to instead just move the note element right outside the ref element. But it may be that we'll want to update the schema to allow this case.