Document the CARGO_CFG_* environment variables - fixes #4302
Document the CARGO_CFG_* environment variables - fixes #4302

tshepang/cargo,tshepang/cargo,rust-lang/cargo,rust-lang/cargo,tsurai/cargo,vojtechkral/cargo,achanda/cargo,tsurai/cargo,tsurai/cargo,achanda/cargo,vojtechkral/cargo,tshepang/cargo,nipunn1313/cargo,eminence/cargo,rustyhorde/cargo,achanda/cargo,vojtechkral/cargo,eminence/cargo,rustyhorde/cargo,rust-lang/cargo,eminence/cargo,rustyhorde/cargo,nipunn1313/cargo,tshepang/cargo,rust-lang/cargo,nipunn1313/cargo,eminence/cargo,rustyhorde/cargo,vojtechkral/cargo,achanda/cargo,tsurai/cargo,nipunn1313/cargo,tsurai/cargo