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).
datum -> countable, one piece of data
datums -> countable, a concrete collection of "datum"s with an emphasis on the fact that it is a bunch of individual "datum"s
data -> non-countable, referring to the general idea of a collection of data without any concrete embodiment

/me bucking prescriptivist thinking since 1907