RT3511: doc fix; req default serial is random
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: Tim Hudson <eb0be8e447f673b41b10eda09d63cd72526b0a42@openssl.org>

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl