[cvas22]

Refs #565, #574 and #628.

All three come from inserting a subdoc with {{ sd }}. A subdoc is a list of paragraphs, so with the plain tag they land inside the text of the paragraph that holds the tag. That leaves an empty paragraph behind (#574, #628), and next to a bookmark or formatted text it makes Word refuse the file (#565). The {{p sd }} tag avoids it, but the docs only show it in passing. This adds a short paragraph about it to the sub-documents section.

Notes

Docs only. Making the plain tag work would mean changing how subdocs are rendered, which I left alone.