dt-bindings: input: syna,rmi4: Document syna,rmi4-s3706b

Mostly irrelevant for authentic Synaptics touchscreens, but very important
for applying workarounds to cheap TS knockoffs.

These knockoffs work well with the downstream driver, and since the user
has no way to distinguish them, later in this patch set, we introduce
workarounds to ensure they function as well as possible.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David Heidelberg <david@ixit.cz>
Link: https://patch.msgid.link/20260523-synaptics-rmi4-dt-v2-1-0645122babdc@ixit.cz
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>