Clarify AbstractBeanDefinition.getBeanClass() return semantics

Closes gh-22960

spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractBeanDefinition.java

@@ -397,10 +397,13 @@ public abstract class AbstractBeanDefinition extends BeanMetadataAttributeAccess
 	}
 
 	/**
-	 * Return the class of the wrapped bean, if already resolved.
-	 * @return the bean class, or {@code null} if none defined
+	 * Return the class of the wrapped bean (assuming it is resolved already).
+	 * @return the bean class (never {@code null})
 	 * @throws IllegalStateException if the bean definition does not define a bean class,
-	 * or a specified bean class name has not been resolved into an actual Class
+	 * or a specified bean class name has not been resolved into an actual Class yet
+	 * @see #hasBeanClass()
+	 * @see #setBeanClass(Class)
+	 * @see #resolveBeanClass(ClassLoader)
 	 */
 	public Class<?> getBeanClass() throws IllegalStateException {
 		Object beanClassObject = this.beanClass;
@@ -416,6 +419,9 @@ public abstract class AbstractBeanDefinition extends BeanMetadataAttributeAccess
 
 	/**
 	 * Return whether this definition specifies a bean class.
+	 * @see #getBeanClass()
+	 * @see #setBeanClass(Class)
+	 * @see #resolveBeanClass(ClassLoader)
 	 */
 	public boolean hasBeanClass() {
 		return (this.beanClass instanceof Class);