add a note that calling strerror(3) will make return value broken.

kame/kame/libipsec/ipsec_strerror.3

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.\"     $Id: ipsec_strerror.3,v 1.2 1999/09/21 03:49:19 itojun Exp $
+.\"     $Id: ipsec_strerror.3,v 1.3 2000/01/16 07:30:36 itojun Exp $
 .\"
 .Dd May 6, 1998
 .Dt IPSEC_STRERROR 3
@@ -50,6 +50,19 @@ which is used to pass error code from IPsec policy manipulation library
 to user program.
 .Fn ipsec_strerror
 can be used to obtain error message string for the error code.
+.Pp
+The array pointed to is not to be modified by the program.
+Since
+.Fn ipsec_strerror
+uses
+.Xr strerror 3
+as underlying function, calling
+.Xr strerror 3
+after
+.Fn ipsec_strerror
+would make the return value from
+.Fn ipsec_strerror
+invalid, or overwritten.
 .\" 
 .Sh RETURN VALUES
 .Fn ipsec_strerror