document limitation of LazyToOne(NO_PROXY) to non-polymorphic associations

we can roll back this commit if/when we fix the limitation, as proposed:

    https://github.com/hibernate/hibernate-orm/discussions/5591

hibernate-core/src/main/java/org/hibernate/annotations/LazyToOne.java

@@ -49,6 +49,10 @@
  * {@code instanceof} and {@link Object#getClass()} work as
  * normal, since the concrete type of the entity instance is
  * known immediately. This is usually bad tradeoff.
+ * <p>
+ * <strong>Currently, Hibernate does not support
+ * {@code LazyToOne(NO_PROXY)} for polymorphic associations,
+ * and instead falls back to using a proxy!</strong>
  *
  * @author Emmanuel Bernard
  */

hibernate-core/src/main/java/org/hibernate/annotations/LazyToOneOption.java

@@ -52,6 +52,10 @@ public enum LazyToOneOption {
 	 * </ul>
 	 * Bytecode enhancement is required. If the class is not
 	 * enhanced, this option is equivalent to {@link #PROXY}.
+	 * <p>
+	 * <strong>Currently, Hibernate does not support this setting
+	 * for polymorphic associations, and instead falls back to
+	 * {@link #PROXY}!</strong>
 	 */
 	NO_PROXY
 }