* docx: ignore w:lnNumType without w:countBy (#6209)
* DOCX: record the w:lnNumType rule in the parity docs (#6209)
A row for `w:lnNumType` (§17.6.8), its status line and a revision entry in
`docs/file-format-parity.md`, the progress-log bullet in `ROADMAP.md`, a comment in
the reader saying why a missing `w:countBy` leaves numbering off, and a save check in
the zero-interval test.