clarify what time is encoded in the sync protocol; add receiver example

[Poofjunior]

The sync protocol docs are a little vague on whether the current or previous second is encoded in the dispatched packet. In practice, it looks like the previous second is being dispatched, not the current second. We have confirmed this behavior with a Synchronizer board and Behavior board.

There is no example of how to unpack the timestamp from the received 4 bytes, so I added one.

[glopesdev]

Feedback from SRM meeting:

• The sync clock should communicate the next second from delivery of the message
• @filcarv Can we check if the current harp core is implementing this spec?
• Should we fix the core implementation or change the standard?
• @filcarv mentioned it might be hard to fix the core, can you elaborate?