enc(1): document that AEAD is not and will not be supported
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 <c04971a99e5a9ee80eaab4b1deb37e845b0bd697@openssl.org>
Reviewed-by: Andy Polyakov <015edf36635e030d5e009e08808a59984e629740@openssl.org>
Reviewed-by: Paul Dale <bc4a6b4cd7a8395c734c8eb5aef04a9c30188c85@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/5048)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl