Clarify that arc does not support /sys/rotation

[Dave Riedstra]

From https://github.com/monome/docs/blob/gh-pages/serialosc/osc.md#sys:

/sys/rotation i

rotate the monome by degrees, where degrees is one of 0, 90, 180, 270. this replaces /cable

It's unclear whether this includes the arc, but according to monome/serialosc#55 (comment) arc is not supported.