SwFmtNoBalancedColumns: add doxygen comment

Change-Id: Ifc538d8a51a333e79cec0312ab8cf4541abba7af
DOCX import: set DontBalanceTextColumns=true for the last section ...

... if it has multiple columns.

See wwSectionManager::InsertSegments() for the related binary import
code which already did this.

Change-Id: I919f585bd864db748cd349e01789ec7805f031a1
CppunitTest_sw_ooxmlimport: allow running a single test

Change-Id: I494cc676f384edbac9f64c1bc02b14cf7973d0d8