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).
@jebeck so, just to make sure I understand the full extent, you are suggesting that we also add

`insulinCarbRatio` - a number
`insulinSensitivity` - a number
`targetBG` - any object that can be used as a `targetBG` on a settings event

Is that everything and true?