Add note to docs about configuration of Log4j 2's JDK logging adapter

Closes gh-11660

Author: wilkinsona

spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -1653,6 +1653,11 @@ requirements (e.g. including having Tomcat use `java.util.logging` but configuri
 output using Log4j 2). See the Actuator Log4j 2 samples for more detail and to see it in
 action.
 
+NOTE: To ensure that debug logging performed using `java.util.logging` is routed into
+Log4j 2, configure its https://logging.apache.org/log4j/2.0/log4j-jul/index.html[JDK
+logging adapter] by setting the `java.util.logging.manager` system property to
+`org.apache.logging.log4j.jul.LogManager`.
+
 
 
 [[howto-configure-log4j-for-logging-yaml-or-json-config]]