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 <1910231d24f125aceac7850dc92279eee8d52323@ncp-e.com>
Reviewed-by: Rich Salz <c04971a99e5a9ee80eaab4b1deb37e845b0bd697@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6802)

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