bug symfony#6634 Update custom_constraint.rst (axelvnk)

This PR was submitted for the 3.1 branch but it was merged into the 2.7 branch instead (closes symfony#6634).

Discussion
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Update custom_constraint.rst

Clear confusion about defining a validator as a service and using it with a custom constraint

Commits
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6ae9022 Update custom_constraint.rst

Author: weaverryan

cookbook/validation/custom_constraint.rst

@@ -167,7 +167,7 @@ Constraint Validators with Dependencies
 If your constraint validator has dependencies, such as a database connection,
 it will need to be configured as a service in the Dependency Injection
 Container. This service must include the ``validator.constraint_validator``
-tag and may include an ``alias`` attribute:
+tag and should include an ``alias`` attribute to be used in the validatedBy method of your validator class:
 
 .. configuration-block::
 
@@ -203,6 +203,9 @@ the constraint, with ``Validator`` appended. You can override this in your const
         return 'Fully\Qualified\ConstraintValidator\Class\Name'; // or 'alias_name' if provided
     }
 
+Make sure to use the 'alias_name' when you have configured your validator as a service. Otherwise your validator class
+will be simply instantiated without your dependencies.
+
 Class Constraint Validator
 ~~~~~~~~~~~~~~~~~~~~~~~~~~