Improve invokeOnCompletion and invokeOnCancellation API

[qwwdfsad]

• Revisit documentation, explicitly spell out the purpose of the API, samplify
• Mark it as delicate API
• Short-cut it if necessary (Investigate possible shortcuts for coroutineContext.job.invokeOnCompletion { if (it != null) ... } #3259)
• Shift invokeOnCompletion doesn't provide thread guarantees and doesn't allow to suspend #3505 and CancellableContinuation.invokeOnCancellation should accept a suspend callback #4154 towards a different API shape

[qwwdfsad]

Also, we have #3065 which is similar to this one, but in a slightly different scope