Digging into some assembly file in order to get information about the
kuser helpers is not that convivial.  Let's move that information to
a better formatted file in Documentation/arm/ and improve on it a bit.

Thanks to Dave Martin <dave.martin@linaro.org> for the initial cleanup and
clarifications.
Signed-off-by: NNicolas Pitre <nicolas.pitre@linaro.org>
Acked-by: NDave Martin <dave.martin@linaro.org>