[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.

what do you think of using a sidebar instead? E.g. `.. sidebar:: Using other Values To Validate a Property` (I'm not happy with the title though, if someone can suggest something better: please use that instead)