Auto merge of #8809 - roblabla:patch-1, r=ehuss
Auto merge of #8809 - roblabla:patch-1, r=ehuss

Document platform-specific build-dependencies

It turns out that `target.'cfg()'.build-dependencies` are a currently stable but undocumented feature. This PR mentions it in the cargo reference.

tshepang/cargo,eminence/cargo,tshepang/cargo,eminence/cargo,tshepang/cargo,rust-lang/cargo,rust-lang/cargo,nipunn1313/cargo,rustyhorde/cargo,nipunn1313/cargo,nipunn1313/cargo,rust-lang/cargo,rustyhorde/cargo,eminence/cargo,tshepang/cargo,eminence/cargo,rustyhorde/cargo,rustyhorde/cargo,nipunn1313/cargo,rust-lang/cargo