minor #20314 [Validator] feat(when constraint): add context variable (KoNekoD)

This PR was merged into the 7.2 branch.

Discussion
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[Validator] feat(when constraint): add context variable

Issue: #20312

Commits
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3df3849 feat(when constraint): add context variable

Author: javiereguiluz

reference/constraints/When.rst

@@ -163,14 +163,16 @@ validation of constraints won't be triggered.
 To learn more about the expression language syntax, see
 :doc:`/reference/formats/expression_language`.
 
-Depending on how you use the constraint, you have access to 1 or 2 variables
+Depending on how you use the constraint, you have access to 1 or 3 variables
 in your expression:
 
 ``this``
     The object being validated (e.g. an instance of Discount).
 ``value``
     The value of the property being validated (only available when
     the constraint is applied to a property).
+``context``
+    This is the context that is used in validation
 
 The ``value`` variable can be used when you want to execute more complex
 validation based on its value: