usb: fix the comment with regards to DocBook

The USB gadget documentation is not at DocBook anymore.
The main file was converted to ReST, and stored at
Documentation/driver-api/usb/gadget.rst, but there are
still several plain text files related to gadget under
Documentation/usb.

So, be generic and just mention documentation
without specifying where it is.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>