Background: syntactically, references to example list items
can't be distinguished from citations; we only know which they
are after we've parsed the whole document (and this is resolved
in the `runF` stage).
This means that pandoc's calculation of `citationNoteNum`
can sometimes be wrong when there are example list references.
This commit partially addresses #6836, but only for the case
where the example list references refer to list items defined
previously in the document.
