Warn that pidfile() will not have the expected removal behaviour if _exit()

is called.

src/lib/libutil/pidfile.3

@@ -1,4 +1,4 @@
-.\"	$OpenBSD: pidfile.3,v 1.3 2001/10/01 06:18:14 mpech Exp $
+.\"	$OpenBSD: pidfile.3,v 1.4 2001/12/01 23:47:37 miod Exp $
 .\"	$NetBSD: pidfile.3,v 1.2 2001/04/12 22:34:31 sommerfeld Exp $
 .\"
 .\" Copyright (c) 1999 The NetBSD Foundation, Inc.
@@ -78,3 +78,12 @@ If
 is called multiple times with different
 .Ar basename ,
 only the last pidfile will be removed upon exit.
+.Pp
+.Fn pidfile
+uses
+.Fn atexit
+to ensure the pidfile is unlinked at program exit.
+However, programs that use the
+.Fn _exit
+function (for example, in signal handlers)
+will not trigger this behaviour.