Adjustments to documentation

This PR includes all adjustments to the documentation listed from

https://trello.com/c/lAGH6360/159-improve-data-model-documentation

@jh-bate @jebeck @kentquirk @nicolashery @ianjorgensen 

Please give this a once over, see if there is anything that could be explained better or just differently.  If you can think of an example that you had a question about what would happen, providing that would also be useful (I can fill in what will actually happen).
...aaand also the target high/low or center-of-range. That's also in the new tooltips, also reported in payloads as `targetHigh` and `targetLow` for Carelink, `target_bg` and/or `bg_correction_threshold` for Insulet.