DOC export: fix nested comments

For one, replace m_nLastRangeStartPos with a map, so that we can keep
track of the start positions when multiple comments are open at the same
time.

For another, sort PlcfAtnBkl and PlcfAtnBkf by position as Word requires
it; with non-nested comments such explicit sorting wasn't necessary.
This also required building two maps, so that we can know what
PlcfAtnBkl index to reference in PlcfandRef, and what PlcfAtnBkf index
to reference in PlcfAtnBkl after sorting.

Change-Id: I2d7096b0c68a6b327efc4b5593ac96cdd297b3bd