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).
First, a point of correction in what you wrote above. The second point should not reference target BG or current BG. Carb bolus calculation only uses I:C ratio across all pumps, AFAIK. (But if you need a correction, as per point one, the wizard will add that value, positive or negative, to the calculated carb bolus.)

For pumps that use the target + range model instead of the high/low model (I believe Animas and Tandem are both such pumps, where I mention vendors in the following, take it all with a prefix of "IIRC"), the steps are as follows:
1. 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 currently within the range (defined as a +/- from target), then don't suggest a correction. If you are below the target - range, a negative correction will be automatically added (Animas) or not (Tandem, much to my dismay).
2. Calculate the appropriate bolus for the carbohydrates you entered using the (appropriate) I:C ratio.
3. Add 1 & 2 and suggest that as the total suggested dose.