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).
Yes @cheddar that's everything and true.

@kentquirk I only partially followed the e-mail discussion about the target range before, now that I've looked at the data I understand a bit better. Definitely willing to dig into it and see exactly how it works. (This also *should* be in the (consumer) manual, one would hope.) I suspect that what we have is the target +/- range model except the range is 0. That is, as long as you are above the threshold, the bolus calculator calculates the dose to get you exactly to the target.) We may need to expand the possible object types for target range to include the threshold(s).