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).
Sure, this was a personal thing of me not thinking that "insulin on board" was readily apparent when I first heard of it (is it the amount of insulin left in the pump?  the amount in the tubing that will be delivered or thrown away? the amount that dripped out of the stomach and onto a board?)

I do now completely know what it means and agree that it seems to be prevalent, so I won't fight it if people want to just use insulinOnBoard.