Auto merge of #2871 - QuiltOS:doc-build-deps, r=alexcrichton
Auto merge of #2871 - QuiltOS:doc-build-deps, r=alexcrichton

Better rational for build-dependencies and other kinds being disjoint.

The old reason was bad because nothing prevents normal dependencies from
being built before the build script.

sbeckeriv/cargo,tsurai/cargo,sbeckeriv/cargo,rustyhorde/cargo,tshepang/cargo,rust-lang/cargo,vojtechkral/cargo,tshepang/cargo,AndiDog/cargo,brson/cargo,eminence/cargo,brson/cargo,nipunn1313/cargo,achanda/cargo,sbeckeriv/cargo,brson/cargo,AndiDog/cargo,achanda/cargo,rust-lang/cargo,vojtechkral/cargo,nipunn1313/cargo,achanda/cargo,tshepang/cargo,eminence/cargo,AndiDog/cargo,eminence/cargo,vojtechkral/cargo,rustyhorde/cargo,brson/cargo,rust-lang/cargo,tsurai/cargo,tsurai/cargo,tsurai/cargo,durka/cargo,nipunn1313/cargo,nipunn1313/cargo,rustyhorde/cargo,durka/cargo,rust-lang/cargo,tsurai/cargo,tshepang/cargo,AndiDog/cargo,rustyhorde/cargo,durka/cargo,vojtechkral/cargo,brson/cargo,sbeckeriv/cargo,achanda/cargo,AndiDog/cargo,durka/cargo,eminence/cargo