i2c: mux: Add more warnings to i2c-arb-gpio-challenge docs

This adds some more warnings to the i2c-arb-gpio-challenge docs to
help encourage people not to use it in their designs unless they have
no choice.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>