`Channel.receiveAsFlow` doesn't document the prompt cancellation guarantee

[dkhalanskyjb]

Nothing in the docs says that channel elements can be lost (and the resources can be leaked) if collection gets cancelled, but it can happen.

        val channel = Channel<Int>(onUndeliveredElement = {
            println("Undelivered element: $it")
        })
        val totalElements = 100
        val expectedElements = AtomicInteger(totalElements)
        launch(Dispatchers.Default) {
            repeat(totalElements) {
                channel.send(it)
            }
            channel.close()
        }
        while(!channel.isClosedForReceive) {
            coroutineScope {
                val job = launch {
                    channel.receiveAsFlow().collect {
                        expectedElements.decrementAndGet()
                    }
                }
                launch(Dispatchers.Default) {
                    job.cancel()
                }
            }
        }
        assertEquals(0, expectedElements.get())

One of the runs prints:

Undelivered element: 38
Undelivered element: 44
Undelivered element: 46
Undelivered element: 49
Undelivered element: 51
Undelivered element: 53
Undelivered element: 55
Undelivered element: 62
Undelivered element: 66
Undelivered element: 68
Undelivered element: 72
Undelivered element: 76
Undelivered element: 78
Undelivered element: 82
Undelivered element: 85
Undelivered element: 87
Undelivered element: 89
Undelivered element: 91
Undelivered element: 93
Undelivered element: 95
Undelivered element: 97

Expected :0
Actual   :21