Document OPENSSL_ENGINES environment variable

In apps/engine.pod and crypto/ENGINE_add.pod

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4956)

(cherry picked from commit c81c38cb)