message: Document platform-specific build-dependencies
  It turns out that `target.'cfg()'.build-dependencies` are a currently stable but undocumented feature.
diff: @@ -346,6 +346,19 @@ manifest:
  cc = "1.0.3"
  ```
  +
  +You can also have target-specific build dependencies by using
  +`build-dependencies` in the target section header instead of `dependencies`. For
  +example:
  +
  +```toml
  +[target.'cfg(unix)'.build-dependencies]
  +cc = "1.0.3"
  +```
  +
  +In this case, the dependency will only be built when the host platform matches the
  +specified target.
  +
  The build script **does not** have access to the dependencies listed
  in the `dependencies` or `dev-dependencies` section. Build
  dependencies will likewise not be available to the package itself
  