Document return types of API methods

[capnrefsmmat]

For example, the pub_covidcast() documentation just says it returns a tibble, with no further information on what's in that tibble. At the least, we should say what each row represents (one observation of one geo_value at one time), and perhaps link to the main API documentation that states what columns there will be and what they contain.

Basically, if a user pulls a data frame from one of these methods, it should be easy to find what's in it. Yes, there's a link to the API docs at the top of most documentation pages, but we should use the sections to make things easy to find; and simple things (like what each row represents) can be put into the documentation directly.

[nmdefries]

Maybe good for @tinatownes to work on.