Document boundary/break

## Compiler version

3.3.0

## Expectation

http://dotty.epfl.ch/docs/reference/dropped-features/nonlocal-returns.html still mentions the old `scala.util.control.NonLocalReturns`, it should talk about `boundary/break` - we can reuse the scaladoc at http://dotty.epfl.ch/api/scala/util/boundary$.html#