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
I've used the same wording as used for option `traverse`.
Personally I also don't like that sentence, and I think that description of both options should be changed (in separate PR?).