Documentation usb serial: fixed how to provide vendor and product id

While trying to test a Cinterion GSM/GPS/3G module I had reconfigured
the USB interface by mistake and therefore needed to run a different
USB driver than CDC-ACM. It turned out that I need the "usbserial" driver.

This file is an official description how to use it:
Documentation/usb/usb-serial.txt

But it is outdated. The parameters vendor= and product= have been
superseded by a /sys interface.

Here was the solution:

https://bbs.archlinux.org/viewtopic.php?id=175499

  insmod usbserial vendor=0x#### product=0x####

becomes (first #### is vendor, second is product)

  modprobe usbserial
  echo #### #### >/sys/bus/usb-serial/drivers/generic/new_id

This patch changes the documentation file to describe the modern variant.
Please note that the old one still works (if compiled as module).
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Marek Belisko <marek@goldelico.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/usb/usb-serial.txt

@@ -465,12 +465,14 @@ Generic Serial driver
   device, and does not support any kind of device flow control. All that
   is required of your device is that it has at least one bulk in endpoint,
   or one bulk out endpoint. 
-  
-  To enable the generic driver to recognize your device, build the driver
-  as a module and load it by the following invocation:
+
+  To enable the generic driver to recognize your device, provide
+	echo <vid> <pid> >/sys/bus/usb-serial/drivers/generic/new_id
+  where the <vid> and <pid> is replaced with the hex representation of your
+  device's vendor id and product id.
+  If the driver is compiled as a module you can also provide one id when
+  loading the module
 	insmod usbserial vendor=0x#### product=0x####
-  where the #### is replaced with the hex representation of your device's
-  vendor id and product id.
 
   This driver has been successfully used to connect to the NetChip USB
   development board, providing a way to develop USB firmware without