Add example derivation rules

Author: popescu-v

khiops/core/dictionary.py

@@ -977,6 +977,9 @@ class Variable:
         Type complement for the ``Structure`` type. Set to "" for other types.
     rule : str
         Derivation rule. Set to "" if there is no rule associated to this variable.
+        Examples:
+        - standard rule: "Sum(Var1, Var2)"
+        - reference rule: "[Identifier]"
     variable_block : `VariableBlock`
         Block to which the variable belongs. Not set if the variable does not belong to
         a block.