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 requested this option because if you're doing uploads, you probably have the text of the previous event handy. But if you are analyzing and annotating existing data (perhaps doing a live update) you may not have the previous event text, but you could query for it, and use the ID.