docs: hwmon: htu31: document debugfs serial_number

Document the debugfs serial_number file exposed by the htu31 driver.

The driver creates a debugfs entry for the sensor serial number, but the documentation currently only describes the sysfs interface.

Signed-off-by Chen-Shi-Hong <[email protected]>

---
 Documentation/hwmon/htu31.rst | 7 +++++++
 1 file changed, 7 insertions(+)

Documentation/hwmon/htu31.rst +7 -0

index ccde84264643..9ab774dcf65d 100644
@@ -35,3 +35,10 @@ temp1_input:        temperature input
 humidity1_input:    humidity input
 heater_enable:      heater control
 =================== =================
+
+debugfs-Interface
+-----------------
+
+=================== =========================================
+serial_number:      unique serial number of the sensor
+=================== =========================================