Document acknowledgements in transport interface

This is the {diameter, ack} message from a transport process to its
parent that requests that every Diameter request passed to the parent be
matched by a incoming send message, the atom false replacing a message()
in the case that the request is unanswered, allowing the transport to
keep track of the number of outstanding requests.

These were added in commit ca09cf7b, and are used to implement the
message_cb config in diameter_tcp/sctp, documented in commit cefcaa5c.

This commit documents the interface, to make it available to other
transport implementations.

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