Doc multiple java agent extension (#6132) (#6133)

* Doc multiple java agent extension  (#6132)

* polished

examples/extension/README.md

@@ -20,6 +20,8 @@ To add the extension to the instrumentation agent:
           -Dotel.javaagent.extensions=build/libs/opentelemetry-java-instrumentation-extension-demo-1.0-all.jar
           -jar myapp.jar
      ```
+Note: to load multiple extensions, you can specify a directory path for the `otel.javaagent.extensions` value.
+
 ## Embed extensions in the OpenTelemetry Agent
 
 To simplify deployment, you can embed extensions into the OpenTelemetry Java Agent to produce a single jar file. With an integrated extension, you no longer need the `-Dotel.javaagent.extensions` command line option.