CCBC-393: Add API documentation about environment variables

Change-Id: I7fbb4c19275c2ae25620ef618b21cdfb115e8d4f
Reviewed-on: http://review.couchbase.org/38777
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>