=str #16549 doc: Integration with External Services

I guess these ordering rules should be explained in more detail, probably a separate section at the bottom ("Ordering rules of mapAsync and mapAsyncUnordered").

It would be nice to show a random delayed service explicitly completing a future after random sleep, printing the completion event, combining it with a print stage upstream and downstream, and then show the user the resulting sequence. To demonstrate:

 - mapAsync receives A
 - mapAsync receives B
 - Future completes with B'
 - mapAsync receives C
 - Future completes with A'
 - mapAsync emits A'
 - Future completes with C'
 - mapAsync emits B'
 - mapAsync emits C'

And

 - mapAsyncUnordered receives A
 - mapAsyncUnordered receives B
 - Future completes with B'
 - mapAsyncUnordered emits B'
 - mapAsyncUnordered receives C
 - Future completes with A'
 - mapAsyncUnordered emits A'
 - Future completes with C'
 - mapAsyncUnordered emits C'