Auto merge of #4303 - bluetech:doc-env-cfg, r=alexcrichton

Document the CARGO_CFG_* environment variables - fixes #4302

In order to keep the paragraph brief, I have omitted:

- examples of actual variable names - though adding those might help google.
- mention of `rustc --print cfg` - I think the link to the reference is enough.
- reference to the [RFC](https://github.com/rust-lang/rfcs/blob/master/text/1721-crt-static.md#lowering-cfg-values-to-cargo-build-script-environment-variables) - I don't think it's needed.