Document CARGO_CFG_* enviromnent variable

[SimonSapin]

#3243 added them. https://kazlauskas.me/entries/writing-proper-buildrs-scripts.html points out (that they exist! and) that they’re not documented on http://doc.crates.io/environment-variables.html

[alexcrichton]

I believe this is a duplicate of #4041, so closing in favor of that issue.