mention how HZ affects us

lib/libc/sys/nanosleep.2

@@ -1,4 +1,4 @@
-.\"	$NetBSD: nanosleep.2,v 1.14 2013/12/25 00:58:50 dholland Exp $
+.\"	$NetBSD: nanosleep.2,v 1.15 2014/02/01 22:42:45 christos Exp $
 .\"
 .\" Copyright (c) 1986, 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -29,7 +29,7 @@
 .\"
 .\"     @(#)sleep.3	8.1 (Berkeley) 6/4/93
 .\"
-.Dd October 1, 2012
+.Dd February 1, 2014
 .Dt NANOSLEEP 2
 .Os
 .Sh NAME
@@ -163,3 +163,15 @@ The
 .Fn clock_nanosleep
 function conforms to
 .St -p1003.1j-2000 .
+.Sh BUGS
+The
+.Nx
+kernel is not tickless, so the maximum sleep resolution is determined
+by the value of
+.Dv HZ
+which is by default
+.Dv 100
+in most architectures.
+This means that a request to sleep for less than
+.Dv 10ms
+(1 / HZ seconds), is rounded up to that.