[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.

I like the sidebar - just because it allows us to have that title. It seems to me that the real benefit to this method would be that it allows you to map the error to a specific field. So, how about `Mapping the Error to a Specific Field`?

Btw, really nice description in the paragraph below - I love it!