Document how to configure root logging level

Fixes gh-2872

Author: philwebb

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

@@ -1138,10 +1138,12 @@ are not managed by spring Boot.
 All the supported logging systems can have the logger levels set in the Spring
 `Environment` (so for example in `application.properties`) using
 '`+logging.level.*=LEVEL+`' where '`LEVEL`' is one of TRACE, DEBUG, INFO, WARN, ERROR,
-FATAL, OFF. Example `application.properties`:
+FATAL, OFF. The `root` logger can be configured using `logging.level.root`.
+Example `application.properties`:
 
 [source,properties,indent=0,subs="verbatim,quotes,attributes"]
 ----
+	logging.level.root=WARN
 	logging.level.org.springframework.web=DEBUG
 	logging.level.org.hibernate=ERROR
 ----