Drivers: hv: vmbus: document the VMBus sysfs files

The 4 sysfs files should be stable ABIs to the user space.
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/stable/sysfs-bus-vmbus

+What:		/sys/bus/vmbus/devices/vmbus_*/id
+Date:		Jul 2009
+KernelVersion:	2.6.31
+Contact:	K. Y. Srinivasan <kys@microsoft.com>
+Description:	The VMBus child_relid of the device's primary channel
+Users:		tools/hv/lsvmbus
+
+What:		/sys/bus/vmbus/devices/vmbus_*/class_id
+Date:		Jul 2009
+KernelVersion:	2.6.31
+Contact:	K. Y. Srinivasan <kys@microsoft.com>
+Description:	The VMBus interface type GUID of the device
+Users:		tools/hv/lsvmbus
+
+What:		/sys/bus/vmbus/devices/vmbus_*/device_id
+Date:		Jul 2009
+KernelVersion:	2.6.31
+Contact:	K. Y. Srinivasan <kys@microsoft.com>
+Description:	The VMBus interface instance GUID of the device
+Users:		tools/hv/lsvmbus
+
+What:		/sys/bus/vmbus/devices/vmbus_*/channel_vp_mapping
+Date:		Jul 2015
+KernelVersion:	4.2.0
+Contact:	K. Y. Srinivasan <kys@microsoft.com>
+Description:	The mapping of which primary/sub channels are bound to which
+		Virtual Processors.
+		Format: <channel's child_relid:the bound cpu's number>
+Users:		tools/hv/lsvmbus

MAINTAINERS

@@ -4957,6 +4957,7 @@ F:	drivers/scsi/storvsc_drv.c
 F:	drivers/video/fbdev/hyperv_fb.c
 F:	include/linux/hyperv.h
 F:	tools/hv/
+F:	Documentation/ABI/stable/sysfs-bus-vmbus
 
 I2C OVER PARALLEL PORT
 M:	Jean Delvare <jdelvare@suse.com>