update docs with reference to SSL_set_ex_data

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26361)

doc/man3/SSL_CTX_set_new_pending_conn_cb.pod

@@ -34,6 +34,10 @@ on a subsequent call to SSL_accept_connection().  A nonzero return
 value is treated as success, allowing the new connection to be enqueued to the
 accept queue.
 
+=head1 SEE ALSO
+
+L<SSL_set_ex_data(3)>
+
 =head1 NOTES
 
 Callbacks in QUIC connections have some limitations to them that should be taken