document DateType limitations

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes (symfony/symfony#5227)
| Applies to    | all
| Fixed tickets | #2266 

Relates to Issue 2266. I wasn't sure if it made sense to consolidate this into one note, since the two options are so close, but I kept them separate for now. Feedback welcome.