Leave a message in doc to indicate 0 is not acceptable

[to be squashed]
Reviewed-by: NTim Hudson <tjh@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4767)

doc/man1/req.pod

@@ -247,8 +247,8 @@ to the self signed certificate otherwise new request is created.
 =item B<-days n>
 
 When the B<-x509> option is being used this specifies the number of
-days to certify the certificate for, otherwise it is ignored.
-The default is 30 days.
+days to certify the certificate for, otherwise it is ignored. B<n> should
+be a positive integer. The default is 30 days.
 
 =item B<-set_serial n>