rtc: Document RTC_VL_READ and RTC_VL_CLR ioctls

RTC_VL_READ and RTC_VL_CLR have been introduced in 2012 with commit
0f20b767 ("drivers/rtc/rtc-pcf8563.c: add RTC_VL_READ/RTC_VL_CLR ioctl
feature")

Document them now that they have been unified.

Link: https://lore.kernel.org/r/20191214220259.621996-3-alexandre.belloni@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>