document versions, issues, as_of, and recommendations

[dsweber2]

These aren't clearly described in either the general API docs or in this package.

[rlunde]

I wonder if this is a good thing to work on as we get back to epidatr, as a way of focusing our attention on what we should do next for epidatr (and epidatpy)?

[dsweber2]

Ideally this would be coordinated with the general docs update, but yeah, probably a good idea, I think this was prompted by Evan being confused by the differences

[dshemetov]

Thinking about a "Requesting versioned data" vignette to demonstrate these arguments. Maybe demonstrate some revisions that occurred between two as_of dates. And demonstrate creating an epi_archive from an issues=* query.