message: Add comment explaining dep_kind in compute_deps_custom_build.
diff: @@ -382,6 +382,11 @@ fn compute_deps<'a, 'cfg>(
  ///
  /// The `unit` provided must represent an execution of a build script, and
  /// the returned set of units must all be run before `unit` is run.
  +///
  +/// `dep_kind` is the dependency kind of the package this build script is
  +/// being built for. This ensures that the build script is built with the same
  +/// features the package is built with (if the build script has cfg!()
  +/// checks).
  fn compute_deps_custom_build<'a, 'cfg>(
  unit: &Unit<'a>,
  dep_kind: DepKind,
  