This patch also adds information about the option "Delete unused backgrounds" that is also available in the referenced dialog.
Change-Id: I25ffea0acd456174181176375e1ab5075e2a17f7
Reviewed-on: https://gerrit.libreoffice.org/c/help/+/121771
Tested-by: Jenkins
Reviewed-by: Ilmari Lauhakangas <ilmari.lauhakangas@libreoffice.org>