docs: dt: writing-bindings: Document discouraged instance IDs

Document established Devicetree bindings maintainers review practice:
instance indexes, either as properties or as custom new OF alias, are
not accepted.  Recommended way is to use, depending on the
situation/hardware: different compatible, cell arguments or syscon
phandle arguments.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250713-dt-bindings-docs-v2-3-672c898054ae@linaro.org


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