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).
what about 

```
{ "target": 100, "high": 120, "start": 0 }
```

From your description it sounds like `correctionThreshold` is the same meaning as `high`.  Or what was the semantic difference between the two names?