Documentation: power: rt9756: Document exported sysfs entries

Document the settings exported by rt9756 charger driver through sysfs
entries:

- watchdog_timer
- operation_mode

Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Link: https://patch.msgid.link/5138d4aa760d6ff81a5296cdf010a447056abce4.1761894605.git.cy_huang@richtek.com
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

Author: ChiYuan Huang

Documentation/ABI/testing/sysfs-class-power-rt9756

@@ -0,0 +1,30 @@
+What:		/sys/class/power_supply/rt9756-*/watchdog_timer
+Date:		Dec 2025
+KernelVersion:	6.19
+Contact:	ChiYuan Huang <cy_huang@richtek.com>
+Description:
+		This entry shows and sets the watchdog timer when rt9756 charger
+		operates in charging mode. When the timer expires, the device
+		will disable the charging. To prevent the timer expires, any
+		host communication can make the timer restarted.
+
+		Access: Read, Write
+
+		Valid values:
+		- 500, 1000, 5000, 30000, 40000, 80000, 128000 or 255000 (milliseconds),
+		- 0: disabled
+
+What:		/sys/class/power_supply/rt9756-*/operation_mode
+Date:		Dec 2025
+KernelVersion:	6.19
+Contact:	ChiYuan Huang <cy_huang@richtek.com>
+Description:
+		This entry shows and set the operation mode when rt9756 charger
+		operates in charging phase. If 'bypass' mode is used, internal
+		path will connect vbus directly to vbat. Else, default 'div2'
+		mode for the switch-cap charging.
+
+		Access: Read, Write
+
+		Valid values:
+		- 'bypass' or 'div2'