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).
Should we call it "InsulinOnBoard" or "IOB"? Both Asante and Insulet call it IOB, and I believe Medtronic does as well. At the very least, we should put IOB in the text here so that people who search for it will find it.