minor #6299 Removed True and False constraints from reference (edefimov)

This PR was squashed before being merged into the 3.0 branch (closes #6299).

Discussion
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Removed True and False constraints from reference

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 3.0+
| Fixed tickets |
This PR simply removes _True_, _False_, _Null_ constraints from reference list.

Commits
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dcb5a54 Removed True and False constraints from reference

Author: xabbuh

reference/constraints.rst

@@ -9,11 +9,8 @@ Validation Constraints Reference
    constraints/Blank
    constraints/NotNull
    constraints/IsNull
-   constraints/Null
    constraints/IsTrue
-   constraints/True
    constraints/IsFalse
-   constraints/False
    constraints/Type
 
    constraints/Email