Documentation Enhancement: clarify the case which "error-channel" works

[tonny1983]

Considering the following two cases:
Case 1: An exception is thrown in the caller's thread (synchronous processes);
Case 2: An exception is thrown in a latter thread after the caller puts a message (asynchronous processes)

In Case 1, the exception will be thrown to the caller. Meanwhile In Case 2, the caller's thread will return immediately when it puts the message, and therefore it never knows what will happen further. So, the message will be routed to the default errorChannel or the channel defined by the errorChannel value in the message's header only in the case.

It is a little confusing that the document does not clarify the condition and there have been already many related questions.