Corrected inconsistencies in the Javadoc.

While reading through the code I found some minor errors/inconsistencies in the Javadoc of ChannelHandler and ChannelHandlerContext. The comments were still talking about attachements and not the new AttributeKeys. Let me know what you think and if I used the correct wording. Thanks.

PS: The Javadoc in HTML: https://dl.dropboxusercontent.com/u/48211751/doc_fixes/index.html