jurt: fix some javadoc errors for JDK 8

Change-Id: I998f5796d7a5f10f790a1e861b741c54d0f62c19
Reviewed-on: https://gerrit.libreoffice.org/11191
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>