sw/qa/ooxmltok: explain the fieldmark test a bit more
sw/qa/ooxmltok: explain the fieldmark test a bit more

Change-Id: Idbfdc440053c5c4cb6c015e49bc20733e72fda72

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core