Add EVP_AEAD_CTX_init(3) manpage to document the new(ish) AEAD API.

Add EVP_AEAD_CTX_init(3) manpage to document the new(ish) AEAD API.
The "authenticated encryption with additional data" API is used for
ciphers like AES-GCM or ChaCha20-Poly1305.  The manpage is a beginning
and certainly needs more work, especially improvements in the EXAMPLES
section.

Based on agl's source code comments.
Converted from pod to mandoc by schwarze@

OK schwarze@ jsing@