add location of PrettyPrinting property in Expert Config

it is hard to find it without location or searching

Change-Id: I8b8b4b0bc7e0be601ba53c8c21ccb982ad745452
Reviewed-on: https://gerrit.libreoffice.org/6800
Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>