Added documentation for message option of the `True` constraint
minor #3631 Added documentation for message option of the `True` constraint (naitsirch)

This PR was merged into the 2.3 branch.

Discussion
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Added documentation for message option of the `True` constraint

This PR adds the documentation of the option `message` of the `True` constraint.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | none

Commits
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baa67dd Added documentation for message option of the `True` constraint