Improving the JavaDoc of ListDataProvider to explain that the wrapped list should not be modified, and that mutations to the items within the list cannot be detected without called List#set() or ListDataProvider#refresh().

Issue: 7114

Review at http://gwt-code-reviews.appspot.com/1656803

Review by: skybrian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10898 8db76d5a-ed1c-0410-87a9-c151d255dfc7