message: Document copy operators
diff: @@ -70,6 +70,8 @@ class LayoutItem {
  must_wrap_(!tokens_.empty() && tokens_.front().before.break_decision ==
  SpacingOptions::MustWrap) {}
  + // Multiple LayoutFunctionSegments can store copies of the same layout.
  + // The objects are copied mostly in LayoutFunctionFactory::* functions.
  LayoutItem(const LayoutItem&) = default;
  LayoutItem& operator=(const LayoutItem&) = default;
  