Added String support for min and max properties for date field helpers

The behavior of the `min` and `max` properties for date field helper methods is not documented and doesn't necessarily behave the way one might expect them to. Therefor, I'm adding documentation.

That being said, I also wanted to start a dialogue about whether support should be added for passing Strings in as valid values for these options in addition to Date/Time objects. There's support for passing Strings in as values for the `value` option on `DatetimeField` and it's not exactly consistent that you can't pass in a properly formatted string as values for `min` or `max` for those helpers. I'd be happy to investigate if the maintainers thought it would be worthwhile.