enc(1): document that AEAD is not and will not be supported

Note the reasons, including streaming output issues and key/iv/nonce
management issues.

Recommend the use of cms(1) instead.

Fixes #471.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/5048)