[DOC] Please clarify how to set screensaver background in docs

Submitted by Mikhail Kolyada

Assigned to Sean Davis @bluesabre

Link to original bug (#14938)

Description

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It is not really clear, I have new installation and my lock screen picked up some random picture from my collection. I now what my lockscreen looking the same as my desktop background, but have no idea where I am supossed to change this, xfce4-screensaver object in settings-editor has no sign of the backgrond settings, so this remains unclear.