Document boundary/break

[bishabosha]

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#