DocxAttributeOutput::SectionBreaks() previously only handled the
text-text and text-table node transitions; implement support for
table-text to avoid loosing a page break on export for the bugdoc.

(View this commit with whitespace ignored to filter out the noise about
SectionBreaks() now accepting non-text nodes, too.)

Change-Id: Ie8a1575374a207399351635bda8c0c076ce7268d