Documentation: power: rt9471: Document exported sysfs entries

Document the settings exported by rt9471 charger driver through sysfs entries:
- sysoff_enable
- port_detect_enable

Signed-off-by: ChiYuan Huang <[e-mail]>
[update kernel version and date]
Signed-off-by: Sebastian Reichel <[e-mail]>

Documentation/ABI/testing/sysfs-class-power-rt9471

···
+What:		/sys/class/power_supply/rt9471-*/sysoff_enable
+Date:		Feb 2023
+KernelVersion:	6.3
+Contact:	ChiYuan Huang <[e-mail]>
+Description:
+		This entry allows enabling the sysoff mode of rt9471 charger devices.
+		If enabled and the input is removed, the internal battery FET is turned
+		off to reduce the leakage from the BAT pin. See device datasheet for details.
+		It's commonly used when the product enter shipping stage. After entering
+		shipping mode, only 'VBUS' or 'Power key" pressed can make it leave this
+		mode. 'Disable' also can help to leave it, but it's more like to abort
+		the action before the device really enter shipping mode.
+
+		Access: Read, Write
+		Valid values:
+		- 1: enabled
+		- 0: disabled
+
+What:		/sys/class/power_supply/rt9471-*/port_detect_enable
+Date:		Feb 2023
+KernelVersion:	6.3
+Contact:	ChiYuan Huang <[e-mail]>
+Description:
+		This entry allows enabling the USB BC12 port detect function of rt9471 charger
+		devices. If enabled and VBUS is inserted, device will start to do the BC12
+		port detect and report the usb port type when port detect is done. See
+		datasheet for details. Normally controlled when TypeC/USBPD port integrated.
+
+		Access: Read, Write
+		Valid values:
+		- 1: enabled
+		- 0: disabled