fork.2: add _exit(2) xref and spell "ID" consistently

lib/libc/sys/fork.2

@@ -1,4 +1,4 @@
-.\"	$NetBSD: fork.2,v 1.25 2019/09/01 19:40:07 sevan Exp $
+.\"	$NetBSD: fork.2,v 1.26 2022/03/24 01:52:48 gutteridge Exp $
 .\"
 .\" Copyright (c) 1980, 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -117,11 +117,12 @@ This limit is configuration-dependent; or
 the limit
 .Dv RLIMIT_NPROC
 on the total number of
-processes under execution by this user id would be exceeded.
+processes under execution by this user ID would be exceeded.
 .It Bq Er ENOMEM
 There is insufficient swap space for the new process.
 .El
 .Sh SEE ALSO
+.Xr _exit 2 ,
 .Xr execve 2 ,
 .Xr setrlimit 2 ,
 .Xr vfork 2 ,