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).
I believe I understand what the OmniPod does:

* Before taking food into account, calculate a correction bolus to bring you to your target based on IOB and current BG. If your current BG is already below your correction threshold, the answer is zero, unless you have enabled negative corrections, in which case being low might cause a negative correction to be calculated.
* Calculate the bolus necessary to reach your target BG based on carbs entered and your current BG. If a negative correction is enabled, add it.

Can you explain how a range works into it, please?