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>

include/uapi/linux/mmc/ioctl.h

@@ -5,7 +5,10 @@
 #include <linux/types.h>
 
 struct mmc_ioc_cmd {
-	/* Implies direction of data.  true = write, false = read */
+	/*
+	 * Direction of data: nonzero = write, zero = read.
+	 * Bit 31 selects 'Reliable Write' for RPMB.
+	 */
 	int write_flag;
 
 	/* Application-specific command.  true = precede with CMD55 */