docs: dt: Update overlay file extension

Building DTB overlays from .dts files is no longer supported.
Update the documentation to reflect this.

Fixes: 81d36273 ("kbuild: Disallow DTB overlays to built from .dts named source files")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Andrew Davis <afd@ti.com>
Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Link: https://lore.kernel.org/r/ebce4d9591dd0259a636196dda31d40901dc04b0.1738752288.git.geert+renesas@glider.be


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>