docs: usb: rename files to .rst and add them to drivers-api

While there are a mix of things here, most of the stuff
were written from Kernel developer's PoV. So, add them to
the driver-api book.

A follow up for this patch would be to move documents from
there that are specific to sysadmins, adding them to the
admin-guide.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Johan Hovold <johan@kernel.org>
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/ir-usb.c

@@ -16,7 +16,7 @@
  * was written by Roman Weissgaerber <weissg@vienna.at>, Dag Brattli
  * <dag@brattli.net>, and Jean Tourrilhes <jt@hpl.hp.com>
  *
- * See Documentation/usb/usb-serial.txt for more information on using this
+ * See Documentation/usb/usb-serial.rst for more information on using this
  * driver
  */