(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.
minor wording / capitalization fixes
Merge pull request #301 from KevinCorcoran/maint/docs-for-env-cache-endpoint

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