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>