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
Yeah, you're right. I should have written it like this:

> If the constraint is applied on a property that holds an array and the ``deep`` option is enabled, each element in that array will be validated recursively.