docs: dt: writing-bindings: describe typical cases

Add a chapter for caveats or typical mistakes.  Source: Rob Herring's
(Devicetree bindings maintainer) comments on LKML.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220403081849.8051-1-krzysztof.kozlowski@linaro.org