Added hint about attaching the expression constraint to a form field
feature #4086 [Reference][Constraints] Added hint about attaching the expression constraint to a form field (peterrehm)

This PR was submitted for the master branch but it was merged into the 2.4 branch instead (closes #4086).

Discussion
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[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.

Commits
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3023dc3 Added hint about attaching the expression constraint to a form field