Include instructions on how to update extensions on ext management

We should have information somewhere about how to properly update extensions so users can update their extensions without issues (or users are going to run into problems updating).

The proper method was already discussed: http://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=44001

Now we need to either put this in the ACP or make an article and put a link to it in the ACP.