Make the description string for the Contest end date field clearer

[DrewAPicture]

The current description for the End Date field in the Contest Add/Edit form explains that if left empty, it will default to the same date as Start Date. While this is actually true under the hood, what happens is that it's still left blank but considered to be the same as Start Date when calculated.

So need to think about how to best convey that omitting the End Date merely makes the contest a one-day event. Any other date and it's a multi-date event.