Add missing Javadoc

See gh-29574

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/ResponseEntityExceptionHandler.java

@@ -75,6 +75,10 @@ public abstract class ResponseEntityExceptionHandler implements MessageSourceAwa
 		this.messageSource = messageSource;
 	}
 
+	/**
+	 * Get the {@link MessageSource} that this exception handler uses.
+	 * @since 6.0.3
+	 */
 	@Nullable
 	protected MessageSource getMessageSource() {
 		return this.messageSource;

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ResponseEntityExceptionHandler.java

@@ -96,6 +96,10 @@ public abstract class ResponseEntityExceptionHandler implements MessageSourceAwa
 		this.messageSource = messageSource;
 	}
 
+	/**
+	 * Get the {@link MessageSource} that this exception handler uses.
+	 * @since 6.0.3
+	 */
 	@Nullable
 	protected MessageSource getMessageSource() {
 		return this.messageSource;