Improve documentation for specifying env vars

Summary: This syntax is somewhat obscure and awkward, try to document it more completely in the location users are most likely to run into it for the first time.

Test Plan: Generated documentation: https://secure.phabricator.com/file/view/PHID-FILE-ba958fd365e5312d9077/

Reviewers: codeblock, egillth, jungejason, tuomaspelkonen, aran

CC:

Differential Revision: 775