Phrase allocation failure more clearly. ok deraadt@

src/lib/libc/string/strdup.3

@@ -1,4 +1,4 @@
-.\"	$OpenBSD: strdup.3,v 1.21 2014/10/11 14:40:42 schwarze Exp $
+.\"	$OpenBSD: strdup.3,v 1.22 2015/12/01 01:32:48 mmcc Exp $
 .\"
 .\" Copyright (c) 1990, 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -29,7 +29,7 @@
 .\"
 .\"     @(#)strdup.3	8.1 (Berkeley) 6/9/93
 .\"
-.Dd $Mdocdate: October 11 2014 $
+.Dd $Mdocdate: December 1 2015 $
 .Dt STRDUP 3
 .Os
 .Sh NAME
@@ -61,7 +61,7 @@ characters from
 .Fa s .
 The resulting string is always NUL-terminated.
 .Pp
-If insufficient memory is available,
+If the memory allocation fails,
 .Dv NULL
 is returned.
 .Sh EXAMPLES