Document that ENGINE_add_conf_module() was deprecated.

Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12226)

doc/man3/OPENSSL_load_builtin_modules.pod

@@ -44,6 +44,10 @@ None of the functions return a value.
 
 L<config(5)>, L<OPENSSL_config(3)>
 
+=head1 HISTORY
+
+ENGINE_add_conf_module() was deprecated in OpenSSL 3.0.
+
 =head1 COPYRIGHT
 
 Copyright 2004-2018 The OpenSSL Project Authors. All Rights Reserved.