hwmon: (jc42) more helpful documentation

The documentation lists standard numbers and chip names in excruciating
detail, but that's all it does.  To help mere mortals in deciding
whether to enable this driver, mention what this sensor is for and in
which systems it might be found.

Also add a link to the actual JC 42.4 specification.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Cc: stable@kernel.org
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>

Documentation/hwmon/jc42

@@ -51,7 +51,8 @@ Supported chips:
   * JEDEC JC 42.4 compliant temperature sensor chips
     Prefix: 'jc42'
     Addresses scanned: I2C 0x18 - 0x1f
-    Datasheet: -
+    Datasheet:
+	http://www.jedec.org/sites/default/files/docs/4_01_04R19.pdf
 
 Author:
 	Guenter Roeck <guenter.roeck@ericsson.com>
@@ -60,7 +61,11 @@ Author:
 Description
 -----------
 
-This driver implements support for JEDEC JC 42.4 compliant temperature sensors.
+This driver implements support for JEDEC JC 42.4 compliant temperature sensors,
+which are used on many DDR3 memory modules for mobile devices and servers. Some
+systems use the sensor to prevent memory overheating by automatically throttling
+the memory controller.
+
 The driver auto-detects the chips listed above, but can be manually instantiated
 to support other JC 42.4 compliant chips.