docs: clarify Jackson2ObjectMapperBuilderCustomizer javadoc about overriding behavior

Signed-off-by: Seokjae Lee <seok9211@naver.com>

Author: doljae

module/spring-boot-jackson/src/main/java/org/springframework/boot/jackson/autoconfigure/Jackson2ObjectMapperBuilderCustomizer.java

@@ -24,6 +24,9 @@
  * Callback interface that can be implemented by beans wishing to further customize the
  * {@link ObjectMapper} through {@link Jackson2ObjectMapperBuilder} retaining its default
  * auto-configuration.
+ * Note: Depending on which methods of {@link Jackson2ObjectMapperBuilder} are used,
+ * customizations may override or replace parts of the default auto-configuration
+ * instead of merely adding to it.
  *
  * @author Grzegorz Poznachowski
  * @since 4.0.0