Yes, you're right, I've unchecked it. The reason was that I don't really know whether files that get created/modified on RAM disk will always have that attribute set so it seemed safer not to rely on that. As a workaround, I turned off synchronization in RamDisk Configuration tool and added scheduled task for shutdown as described in guideline https://forums.guru3d.com/threads/guide-using-imdisk-to-set-up-ram-disk-s-in-windows-with-no-limit-on-disk-size.356046/ and it seems to work fine so far.
Slow RAM disk synchronization on Windows shutdown
Works for me if I add fork="true" to the java task call in generate-documentation...