dt-bindings: iio: temperature: ltc2983: describe broken mux delay property

The 'adi,mux-delay-config-us' property is broken. It was supposed to
be in us, but the value is actually written directly to the register.
Describe the fact that it is broken and how it actually works.

Signed-off-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20221103130041.2153295-11-demonsingur@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>