Doc: add doc/man7/openssl_user_macros.pod.in

This manual is a start to describe macros that users can use to affect
what symbols are exported by the public header files.

Because the macro OPENSSL_API_COMPAT has a default that's affected by
configuration choices, we must make it a generated manual.

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/7742)