docs: dt: writing-bindings: Extend compatible fallbacks guideline

Extend the guidelines when to use fallback compatibles to cover to
common review responses.  Devices are most likely compatible and should
use fallbacks when having:

1. Compatible programming interface, meaning one is a subset, and Linux
   device drivers can use the subset to correctly match/bind and still
   operate with the subset features.

2. Device variant discovery through some means, like registers.

Devices are incompatible and fallback is not suitable when that
fallback cannot be used by the drivers to match/bind.  In the same time
commit message should clearly explain when the code suggests devices
are compatible, but the binding does not define them as such.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://patch.msgid.link/20260427160739.175451-2-krzysztof.kozlowski@oss.qualcomm.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

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