user guide: document usage of Java system properties
user guide: document usage of Java system properties

Update issue 680
Status: Review
Cc: pekka.klarck

fiuba08/robotframework,fiuba08/robotframework,ldtri0209/robotframework,fiuba08/robotframework,waldenner/robotframework,waldenner/robotframework,ldtri0209/robotframework,ldtri0209/robotframework,fiuba08/robotframework,waldenner/robotframework,ldtri0209/robotframework,waldenner/robotframework,fiuba08/robotframework,ldtri0209/robotframework,waldenner/robotframework