Document platform-specific build-dependencies
Document platform-specific build-dependencies

It turns out that `target.'cfg()'.build-dependencies` are a currently stable but undocumented feature.
eminence/cargo,nipunn1313/cargo,rustyhorde/cargo,rust-lang/cargo,nipunn1313/cargo,rustyhorde/cargo,eminence/cargo,tshepang/cargo,rustyhorde/cargo,rustyhorde/cargo,eminence/cargo,nipunn1313/cargo,rust-lang/cargo,tshepang/cargo,tshepang/cargo,eminence/cargo,rust-lang/cargo,nipunn1313/cargo,tshepang/cargo,rust-lang/cargo