Document structured concurrency

[dkhalanskyjb]

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Draft for now, because the documentation of some other functions should be updated accordingly:

• [x] The remaining entries in CoroutineScope.kt.
• [x] supervisorScope
• [x] Builders.common.kt: launch, async, and the other coroutine builders should document their structured concurrency behaviors.
• [ ] The withTimeout family of functions.
• [ ] produce.
• [ ] Coroutine builders from the reactive integrations.
• [ ] JS+Wasm promise.
• [ ] The future function.

but reviewing the changes should already be okay (I'll introduce further changes in separate commits).

cc @LouisCAD, given your real-world experience and interest in the documentation, your comments on this would be greatly appreciated!