RT3511: doc fix; req default serial is random

RT842, closed back in 2004, changed the default serial number
to be a random number rather than zero.  Finally time to update
the doc
Reviewed-by: NTim Hudson <tjh@openssl.org>

doc/apps/req.pod

@@ -235,8 +235,8 @@ this option outputs a self signed certificate instead of a certificate
 request. This is typically used to generate a test certificate or
 a self signed root CA. The extensions added to the certificate
 (if any) are specified in the configuration file. Unless specified
-using the B<set_serial> option B<0> will be used for the serial
-number.
+using the B<set_serial> option, a large random number will be used for
+the serial number.
 
 =item B<-days n>