add SOURCE section to usb(3)

Reference: /n/sources/patch/applied/doc-man-3-usb
Date: Wed May 18 00:32:49 CES 2005

--- /sys/man/3/usb	Wed May 18 00:32:37 2005
+++ /sys/man/3/usb	Wed May 18 00:32:34 2005
@@ -222,6 +222,13 @@
 USB device data channel for the
 .IR k 'th
 configuration.
+.SH SOURCE
+.B /sys/src/9/pc/usb.h
+.br
+.B /sys/src/9/pc/devusb.c
+.br
+.B /sys/src/9/pc/usbuhci.c
+.br
 .SH "SEE ALSO"
 .IR usb (4),
 .IR usbd (4),