minor #21191 [Validator] Clarify the usage of the value variable for the When constraint (alcohol)

This PR was merged into the 6.4 branch.

Discussion
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[Validator] Clarify the usage of the `value` variable for the When constraint

Fixes #21189

`@stof` does this look alright / reads naturally ?

Commits
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7098dcc Clarify the usage of the `value` variable in the When constraint documentation

Author: javiereguiluz

reference/constraints/When.rst

@@ -173,8 +173,9 @@ 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).
+    Either the object being validated (when the constraint is applied to a class),
+    the value of the property being validated (when applied to a property),
+    or the :doc:`raw value </validation/raw_values>`.
 
 The ``value`` variable can be used when you want to execute more complex
 validation based on its value: