[Validator] Documented the "compared_value_path" placeholder

Author: javiereguiluz

reference/constraints/_comparison-propertypath-option.rst.inc

@@ -8,3 +8,14 @@ It defines the object property whose value is used to make the comparison.
 For example, if you want to compare the ``$endDate`` property of some object
 with regard to the ``$startDate`` property of the same object, use
 ``propertyPath="startDate"`` in the comparison constraint of ``$endDate``.
+
+.. tip::
+
+    When using this option, its value is available in error messages as the
+    ``{{ compared_value_path }}`` placeholder. Although it's not intended to
+    include it in the error messages displayed to end users, it's useful when
+    using APIs for doing any mapping logic on client-side.
+
+    .. versionadded:: 4.4
+
+        The ``{{ compared_value_path }}`` placeholder was introduced in Symfony 4.4.