add location of PrettyPrinting property in Expert Config

it is hard to find it without location or searching

Change-Id: If13b9dfc0b4048f7b1367147f5342f6d069eadb9
Reviewed-on: https://gerrit.libreoffice.org/6799
Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>