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).
Jana -- I'd like your take on that for Insulet. What does this "range" mean? ```bg_correction_threshold``` is the level below which you won't get a correction. There's an absolute low in the eeprom settings below which you won't get a bolus, either. But how would you translate that to a "range"? As far as I can tell, only one number, the target, is ever used by insulet to calculate a bolus.