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>

diff --git a/Documentation/ABI/testing/sysfs-class-power-rt9756 b/Documentation/ABI/testing/sysfs-class-power-rt9756
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index 0000000..c4d6c2b
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+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'