Warn about DateTime#===

Based on discussion in #734 I propose we warn about the change to matching with `#===` in certain circumstances, DateTime being one of them. WDYT.
I think the last clause should be a separate sentence, not simply separated by a comma.  Also, it's probably good to soften this with "We recommend you switch" rather than simply saying "switch":

``` ruby
...this may result in undesired behavior. We recommend you switch to a
`Date` or `Time` object instead.
```