quickly document the flags usbd does take.

Reference: /n/sources/patch/applied/usbdusage
Date: Tue Feb 12 03:22:36 CET 2008
Signed-off-by: quanstro@quanstro.net

--- /sys/man/4/usbd	Tue Feb 12 03:22:06 2008
+++ /sys/man/4/usbd	Tue Feb 12 03:22:05 2008
@@ -4,7 +4,7 @@
 .SH SYNOPSIS
 .B usbd
 [
-.B -dv
+.B -dfDV
 ]
 .SH DESCRIPTION
 .I Usbd
@@ -18,10 +18,24 @@
 .B Usbd
 recognizes the following options:
 .TP
+.B f
+Don't fork.
+.TP
 .B d
+Set USB library debugging bitmask. A value of 1
+sets
+.BI Dbginfo ,
+2 sets
+.BI Dbgfs ,
+4 sets
+.BI Dbgproc ,
+and 8 sets
+.BI Dbgcontrol .
+.TP
+.B D
 Debug; print the bytes in each message sent or received.
 .TP
-.B v
+.B V
 Verbose; print configuration information and device status as they change.
 .SH SOURCE
 .B /sys/src/cmd/usb/usbd
--- /sys/src/cmd/usb/usbd/usbd.c	Tue Feb 12 03:22:12 2008
+++ /sys/src/cmd/usb/usbd/usbd.c	Tue Feb 12 03:22:11 2008
@@ -30,7 +30,7 @@
 static void
 usage(void)
 {
-	fprint(2, "usage: usbd\n");
+	fprint(2, "usage: usbd [-dfDV]\n");
 	threadexitsall("usage");
 }