mention that @NaturalId cannot be null in javadoc (#7370)

as in #7367

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

@@ -70,6 +70,11 @@ import static java.lang.annotation.RetentionPolicy.RUNTIME;
  * Unlike the {@linkplain jakarta.persistence.Id primary identifier}
  * of an entity, a natural id may be {@linkplain #mutable}.
  * <p>
+ * On the other hand, a field or property which forms part of a natural
+ * id may never be null, and so it's a good idea to use {@code @NaturalId}
+ * in conjunction with the Bean Validation {@code @NotNull} annotation
+ * or {@link jakarta.persistence.Basic#optional @Basic(optional=false)}.
+ * <p>
  * The {@link org.hibernate.Session} interface offers several methods
  * that allow an entity instance to be retrieved by its
  * {@linkplain org.hibernate.Session#bySimpleNaturalId(Class) simple}
@@ -78,12 +83,12 @@ import static java.lang.annotation.RetentionPolicy.RUNTIME;
  * NaturalIdCache natural id caching}, then these methods may be able
  * to avoid a database round trip.
  *
- * @author Nicol<EFBFBD>s Lichtmaier
+ * @author Nicolás Lichtmaier
  *
  * @see NaturalIdCache
  */
-@Target( { METHOD, FIELD } )
-@Retention( RUNTIME )
+@Target({METHOD, FIELD})
+@Retention(RUNTIME)
 public @interface NaturalId {
 	/**
 	 * Specifies whether the natural id is mutable or immutable.