w1_therm: adding sysfs-driver-w1_therm doc

Adding a sysfs-driver-w1_therm documentation file in
Documentation/ABI/testing. It describe the onlys sysfs entry of w1_therm
module, based on Documentation/w1/slaves/w1_therm.rst
Signed-off-by: Akira Shimahara <akira215corp@gmail.com>
Link: https://lore.kernel.org/r/20200511203631.410227-1-akira215corp@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-driver-w1_therm

+What:		/sys/bus/w1/devices/.../w1_slave
+Date:		May 2020
+Contact:	Akira Shimahara <akira215corp@gmail.com>
+Description:
+		(RW) return the temperature in 1/1000 degC.
+		*read*: return 2 lines with the hexa output data sent on the
+		bus, return the CRC check and temperature in 1/1000 degC
+		*write* :
+			* '0' : save the 2 or 3 bytes to the device EEPROM
+			(i.e. TH, TL and config register)
+			* '9..12' : set the device resolution in RAM
+			(if supported)
+			* Anything else: do nothing
+		refer to Documentation/w1/slaves/w1_therm.rst for detailed
+		information.
+Users:		any user space application which wants to communicate with
+		w1_term device
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