Mention strtok_r in STANDARDS

POSIX 2001 instead of POSIX.1c suggested by millert@ and jmc@, ok jmc@

src/lib/libc/string/strtok.3

@@ -1,4 +1,4 @@
-.\"	$OpenBSD: strtok.3,v 1.21 2013/06/05 03:39:23 tedu Exp $
+.\"	$OpenBSD: strtok.3,v 1.22 2017/08/31 10:57:47 jca Exp $
 .\"
 .\" Copyright (c) 1988, 1991 The Regents of the University of California.
 .\" All rights reserved.
@@ -31,7 +31,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd $Mdocdate: June 5 2013 $
+.Dd $Mdocdate: August 31 2017 $
 .Dt STRTOK 3
 .Os
 .Sh NAME
@@ -140,6 +140,10 @@ The
 .Fn strtok
 function conforms to
 .St -ansiC .
+The
+.Fn strtok_r
+function conforms to
+.St -p1003.1-2001 .
 .Sh HISTORY
 The
 .Fn strtok