(docs) Fix glitches noticed while backporting recent revisions to 1.6.x

* In 2.1 and up, the events endpoint is unsorted for v3 and v4.
* v3 doesn't support environments, but one line about them snuck in.
* Accidentally let "certname" (a v4-ism) slip into v3/nodes.
* Fix a busted URL and some glitchy header levels.