Better describe meaning/context of endpoints in documentation

[nmdefries]

These are really issues with lack of information in the Epidata doc website. We'll probably end up mirroring info from there, so will need to wait for those to be updated (project planned for this semester, maybe).

• unclear what pub_covid_hosp_facility signal cols represent. There are covariance columns (names end with "_cov")??
• what locations are allowed for pub_flusurv?
• pub_ecdc_ili not clear what are valid region names. Example uses a (lowercase and unabbreviated) country name. online docs don't list allowed regions either, and is basically a stub.
• pub_kcdc_ili not clear what are valid region names. Example uses a (uppercase and abbreviated) acronym -- unclear what it means. online docs don't list allowed regions either, and is basically a stub. Nowhere does it say what the KCDC is or link to any additional info
• Private endpoints need more info about authorization required. Can non-Delphi users get access to the private endpoints? Why are they private?
• hard to tell that pub_nidss_flu has non-US data, has Taiwanese data, and what the regions to request are
• can't tell what pub_meta returns. What API? How is it different than pub_covidcast_meta? Docs and blurb in avail_endpoints need more fleshing out

[nmdefries]

From #212 (comment)

unclear what pub_covid_hosp_facility signal cols represent. There are covariance columns (names end with "_cov")??

_cov = "coverage", not "covariance". And "coverage" in some sense of % facilities reporting the corresponding non-_cov column or something like that. Would need to read upstream docs. I think they do just use that _cov naming upstream though, unfortunate.

[nmdefries]

Maybe good for @tinatownes to work on.