Updated Course End Date info

To be more specific about end dates and certificates.

https://edx-wiki.atlassian.net/secure/RapidBoard.jspa?rapidView=110&view=detail&selectedIssue=DOC-298
OK. I was just thinking in terms of courses like DemoX, but I guess folks wouldn't want a certificate for that course anyway.

We should maybe make this more prominent - turn it into a Note, perhaps? @mhoeber, your thoughts?