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).
No, I think you're right. There's no semantic difference. There could be if a new pump comes on the market that lets you set a `target`, `low`, and `high` (so, like target + range except that the range can be different on either side).

Whether we get the semantics exactly right depends a lot of whether we expect to ever replicate the bolus calculator math in the application. Right now, I don't think we expect to do that, since we get the calculation information (suggested doses, mainly) directly from the wizard events. But if we start supporting more pumps (e.g., Animas), there's no guarantee their wizard events will include that info, so we might have to calculate it ourselves to display it (assuming that if we can get data directly out of an Animas pump we'll have a full settings history to be able to look up the current wizard settings at the time of a bolus, which might be a big assumption...).