Updates the section "Coroutines are light-weight" to clarify its
comparison of the resource-intensiveness of coroutines and threads. The
content is much the same, I've attempted to make it follow the pattern
of the other sections by introducing the conceptual point first, then
presenting the code sample.

This commit also makes the code sample in this section runnable and
omits the surrounding runBlocking block.

- Add some additional text that emphasizes that the example code has a
  small memory footprint.
- Add a note that we should not make the example runnable (since it will
  exhaust the playground heap)
- Remove //sample(Start|End) comments since they are not recognized for
  code that is not runnable.