[Reference][Constraints] Added hint about attaching the expression constraint to a form field

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4
| Fixed tickets | -

As per my discussion with @webmozart a while ago on twitter https://twitter.com/peterrehm/status/434350363940904960 I am now providing the hint about the possibility to add a expression constraint to a form field.

Thank you! I think we have to go through all the code examples we have and make them as consistent as possible. It's hard for users if they have to search through the complete documentation to find where to place the piece of XML they found. But that's a separate issue. For now it's fine!