Document default section and library configuration

It is talked around but not explicitly stated in one part of the
documentation that you should put library configuration lines at the
start of the configuration file.

CLA: trivial

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/3959)