doc: dts: add missing entry to dts/bindings search path

Since SHIELD_DIRS gets added to DTS_ROOT in dts.cmake, any shield
directories are also places where we look for bindings by default.
This feature is not used in upstream Zephyr, but it is supported,
so document it.

Suggested-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>