sigh: updated documentation

src/entt/signal/sigh.hpp

@@ -257,6 +257,10 @@ private:
  * The clear separation between a signal and a sink permits to store the former
  * as private data member without exposing the publish functionality to the
  * users of the class.
+ * 
+ * @warning
+ * Lifetime of a sink must not overcome that of the signal to which it refers.
+ * In any other case, attempting to use a sink results in undefined behavior.
  *
  * @tparam Ret Return type of a function type.
  * @tparam Args Types of arguments of a function type.