Updated Valid constraint reference

| Q              | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3
| Fixed tickets | 

Revised PR #4351 for 2.3 branch
Maybe we can reword this a bit. For example:

> If this constraint is enabled on a property that holds an array, each element in that array will be validated recursively.

What do you think?