Document the new otel.javaagent.logging setting (#2618)

Co-authored-by: Phillip Carter <[email protected]>

content/en/docs/instrumentation/java/automatic/agent-config.md

@@ -79,6 +79,28 @@ extension.
 
 {{% /config_option %}}
 
+### Javaagent logging output
+
+The agent's logging output can be configured by setting the following property:
+
+{{% config_option name="otel.javaagent.logging" %}}
+
+The javaagent logging mode. The following 3 modes are supported:
+
+- `simple`: The agent will print out its logs using the standard error stream.
+  Only `INFO` or higher logs will be printed. This is the default javaagent
+  logging mode.
+- `none`: The agent will not log anything - not even its own version.
+- `application`: The agent will attempt to redirect its own logs to the
+  instrumented application's slf4j logger. This works the best for simple
+  one-jar applications that do not use multiple classloaders; Spring Boot apps
+  are supported as well. The javaagent output logs can be further configured
+  using the instrumented application's logging configuration (e.g. `logback.xml`
+  or `log4j4.xml`). **Make sure to test that this mode works for your
+  application before running it in a production environment.**
+
+{{% /config_option %}}
+
 ## Common instrumentation configuration
 
 Common settings that apply to multiple instrumentations at once.