doc: application specific devicetree overlay file described, app.overlay

Follow-up: #16817, commit af968c26942feef3628fc05293c1cb3584e719a4

The referred commit introduced app.overlay as a general application
devicetree overlay if no specific board overlay is found.

This commit document this behavior in the devicetree howto.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>