Auto merge of #5443 - rtsuk:update-doc-env-var, r=alexcrichton
authorbors <bors@rust-lang.org>
Mon, 30 Apr 2018 18:19:47 +0000 (18:19 +0000)
committerbors <bors@rust-lang.org>
Mon, 30 Apr 2018 18:19:47 +0000 (18:19 +0000)
Update documentation on config via environment variables

Use source replacement as a specific example of the kind
of configuration feature that cannot currently be set by
environmental variables.


Trivial merge