add comments in margins option to PDF export

Change-Id: I0aa253632f32da00631f331c016345cd1fe2d9e5
Reviewed-on: https://gerrit.libreoffice.org/9243
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>