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

Documentation/devicetree/bindings/i2c/i2c-arb-gpio-challenge.txt

@@ -8,6 +8,12 @@ the standard I2C multi-master rules.  Using GPIOs is generally useful in
 the case where there is a device on the bus that has errata and/or bugs
 that makes standard multimaster mode not feasible.
 
+Note that this scheme works well enough but has some downsides:
+* It is nonstandard (not using standard I2C multimaster)
+* Having two masters on a bus in general makes it relatively hard to debug
+  problems (hard to tell if i2c issues were caused by one master, another, or
+  some device on the bus).
+
 
 Algorithm: