Update javadoc of ChannelManagerEvent class

http://www.onehippo.org/library/concepts/channels/channelmanagereventlistener.html

Somewhere on that page is this bit of code:

Channel channel = event.getChannel();
if (channel != null) {
...snip...
}

The code checks for null, but it seems getChannel will never be null. The javadoc should say so, so developers can safely leave the null check out.