Warning about future fail-fast in consumeEach is added to the guide

Author: elizarov

reactive/coroutines-guide-reactive.md

@@ -216,6 +216,12 @@ We have two of them in this code and that is why we see "Begin" printed twice.
 In Rx lingo this is called a _cold_ publisher. Many standard Rx operators produce cold streams, too. We can iterate
 over them from a coroutine, and every subscription produces the same stream of elements.
 
+**WARNING**: It is planned that in the future a second invocation of `consumeEach` method
+on an channel that is already being consumed is going to fail fast, that is
+immediately throw an `IllegalStateException`.
+See [this issue](https://github.com/Kotlin/kotlinx.coroutines/issues/167)
+for details.
+
 > Note, that we can replicate the same behaviour that we saw with channels by using Rx 
 [publish](http://reactivex.io/RxJava/2.x/javadoc/io/reactivex/Flowable.html#publish()) 
 operator and [connect](http://reactivex.io/RxJava/2.x/javadoc/io/reactivex/flowables/ConnectableFlowable.html#connect())