Document BIO_{get,set}_conn_ip_family macros.

Add BIO_get_conn_ip_family and BIO_set_conn_ip_family macros to
util/private.num and document them in BIO_s_connect.pod.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/5007)