DOCX export: handle exact end of paragraph w:sdt tags

Previously every paragraph SDT was closed immediately after the
paragraph end. This commit adds support for having multiple paragraphs
inside an SDT.

A few testcases implicitly tested that such SDT's are lost on save,
adjust the relevant XPath expressions now that this works.

Change-Id: I07802b3e067600b087b7e0f9b2e7b3ba17c3379a