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

Documentation/ABI/testing/rtc-cdev

@@ -33,6 +33,14 @@ Description:
 		  Requires a separate RTC_PIE_ON call to enable the periodic
 		  interrupts.
 
+		* RTC_VL_READ: Read the voltage inputs status of the RTC when
+		  supported. The value is a bit field of RTC_VL_*, giving the
+		  status of the main and backup voltages.
+
+		* RTC_VL_CLEAR: Clear the voltage status of the RTC. Some RTCs
+		  need user interaction when the backup power provider is
+		  replaced or charged to be able to clear the status.
+
 		The ioctl() calls supported by the older /dev/rtc interface are
 		also supported by the newer RTC class framework. However,
 		because the chips and systems are not standardized, some PC/AT