[Validator] Add warning about closure not being cachable

Author: Nyholm

reference/constraints/Callback.rst

@@ -251,6 +251,13 @@ constructor of the Callback constraint::
  }
  }
 
+.. warning::
+
+ Using a ``Closure`` together with annotation configuration will disable the
+ annotation cache for that class/property/methods because ``Closure``s cannot
+ be cached. For best performance, it is recommended to use a static callback
+ method.
+
 Options
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