Add warning about combinatorial explosion to documentation.

src/search/tasks/default_value_axioms_task.cc

@@ -420,6 +420,7 @@ static plugins::TypedEnumPlugin<AxiomHandlingType> _enum_plugin({
          "have cyclic dependencies by setting an empty condition, "
          "indicating the default value can always be achieved for free."
          "For all other derived variables, the negated axioms are computed"
-         "exactly."}
+         "exactly. Note that this can potentially lead to a combinatorial "
+         "explosion"}
     });
 }