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
Suggest - "Once grades and certificates are finalized, all students who were enrolled during the course will see their final grade (as a percentage) on their student dashboards, as well as the score necessary to earn a certificate. For students how have earned a certificate....."

Let me know if it's too wordy!