Documentation improvement about async.await cancelling outer scope

[Tolriq]

After a discussion on slack I discovered that fact (async.await cancelling outer scope) and could not find a proper explanation / really visible point in the current documentation.

https://github.com/Kotlin/kotlinx.coroutines/blob/master/docs/exception-handling.md#exception-handling

Explanations are visible in #763 but I do not think users will take time to read all the issues to find such things.

Since it's a relatively major point as most docs suggest to have a global coroutineContext for the activity or fragment or whatever, not knowing that can generate hard to find issues.

[rocketraman]

Related to this is supervisorScope... the section about supervision in the docs does not provide any examples of various recovery scenarios. For example, if I am doing parallel task decomposition via async inside a supervisorScope, how do I either let every job complete or alternatively cancel all children? Are their other recovery scenarios possible as well?

There is a good example of this here: https://stackoverflow.com/a/53222272/430128