mmc: document 'Reliable Write' bit in uapi header

If we use it this way, people should know about it. Also, replace
true/false with nonzero/zero because the flag is not strictly a bool
anymore.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>