Docx Parser: Produce endnotes.

The parser had been changing both footnotes and endnotes into footnotes. This
hasn't been a problem, because they're later collapsed anyway, but the parser should
maintain as much of the docx structure as is defined, and let the
toplevel reader worry about how to translate it into Pandoc. (This would
be an issue when, as is planned, the docx parser spins off into its
own module.)

The output is the same, so no test change is required.