doc/BN_generate_prime: update doc about other callback values

doc/BN_generate_prime: update doc about other callback values

This here page only documents the callback values 0 to 2, but the
callers of BN_generate_prime_ex() call it with the value 3.

The list of manual pages in the SEE ALSO section was extended with the
output from

    git grep BN_GENCB_call.*[3-9]

while in the doc/man3 directory.

Signed-off-by: Beat Bolli <dev@drbeat.li>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6802)