usbhid.4: Note where debug messages go

PR:		280377
Sponsored by:	The FreeBSD Foundation

Author: emaste

share/man/man4/usbhid.4

@@ -67,6 +67,8 @@ Default is 1.
 Debug output level, where 0 is debugging disabled and larger values increase
 debug message verbosity.
 Default is 0.
+Debug messages are printed on the system console and can be viewed using
+.Xr dmesg 8 .
 .El
 .Sh SEE ALSO
 .Xr ehci 4 ,