(maint) docs for admin API + /environment-cache

* Add docs on the new /environment-cache endpoint.

* Update the config doc with more detail about configuring the admin
  API.

* Minor formatting / wording fixes in the /certificate_status section.
There are many possible ways a response could be unsuccessful; most of them are normal HTTP 4xx errors, nothing specific to do with this endpoint.  Looking at the implementation, there's no special error handling for this endpoint, so it seems like there's not anything else worth mentioning here.