Guaranteed message handling documentation

[nicknezis]

The current set of Heron documentation does not mention any of the message handling details such as "anchoring" and "ack/fail" logic. There are some brief mentions to ack and fail in Interface discussion, but Heron should have documentation similar to the logic described here: https://storm.apache.org/releases/1.2.3/Guaranteeing-message-processing.html