Expand and clarify SSL_CTX_config(3) docs.

Expand and clarify SSL_CTX_config(3) docs.

- Document significant limitations in CONF_module_load_file() in OpenSSL 3.x

- Given the limitations, deëmphasise the use of CONF_module_load_file()
  in SSL_CTX_config(3) documentation, showing an example with the default
  config file instead.

Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28650)