fdo#83395 - improve PDF export dialog labels and tooltips

Change-Id: Idddce2dc81e86cc840da5d6d8891487cee6609d4
Reviewed-on: https://gerrit.libreoffice.org/12081
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>