minor #13279 Update UniqueEntity->message description (scriptibus, Lucas Nothnagel)

This PR was merged into the 3.4 branch.

Discussion
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Update UniqueEntity->message description

The description of the message property implies that it is not possible to change the mapping of the error message.

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Commits
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694a911 Change wording 'in' to 'via'
9dc18b3 Update UniqueEntity.rst

Author: OskarStark

reference/constraints/UniqueEntity.rst

@@ -145,9 +145,9 @@ each with a single field.
 
 **type**: ``string`` **default**: ``This value is already used.``
 
-The message that's displayed when this constraint fails. This message is always
-mapped to the first field causing the violation, even when using multiple fields
-in the constraint.
+The message that's displayed when this constraint fails. This message is by default
+mapped to the first field causing the violation. When using multiple fields
+in the constraint, the mapping can be specified via the `errorPath`_ property.
 
 Messages can include the ``{{ value }}`` placeholder to display a string
 representation of the invalid entity. If the entity doesn't define the