DOCX export: avoid 0 or 1 relativeHeight when it's a real value

The z-order can be any number, Word uses kind of random 32bit integers,
we count from 0. It turns out 0 and 1 may have some special meaning, as
counting from 2 (instead of 0) fixes the visibility the problems in the
bugdoc.

Change-Id: I695a625fc0ab8206cc09896bcf02ff7689f1defc