Add comment to explain decision tree

References #525

(cherry picked from commit ad69686)

src/main/java/com/rabbitmq/client/impl/ChannelN.java

@@ -386,6 +386,10 @@ private void releaseChannel() {
                 Basic.Cancel m = (Basic.Cancel)method;
                 String consumerTag = m.getConsumerTag();
                 Consumer callback = _consumers.remove(consumerTag);
+                // Not finding any matching consumer isn't necessarily an indication of an issue anywhere.
+                // Sometimes there's a natural race condition between consumer management on the server and client ends.
+                // E.g. Channel#basicCancel called just before a basic.cancel for the same consumer tag is received.
+                // See https://github.com/rabbitmq/rabbitmq-java-client/issues/525
                 if (callback == null) {
                     callback = defaultConsumer;
                 }