[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.

Now I got what you guys meant with sidebar. Just updated the PR with the sidebar. I am fine with  "Mapping the Error to a Specific Field"