Polish Undertow access logs contribution

- Apply project’s code formatting and conventions
 - Don’t use the IO and worker thread configuration when creating the
   worker for the AccessLogReceiver. The IO and worker thread
   configuration is for HTTP request processing and a worker in its
   default configuration should be sufficient for the access log
   receiver.
 - Don’t use a temporary directory as the default for the access log
   directory. A temporary directory makes (some) sense for Tomcat as it
   requires a directory for its basedir. Undertow has no such
   requirement and using a temporary directory makes it hard to locate
   the logs. The default has been updated to a directory named logs,
   created in the current working directory.
 - Document the new properties in the application properties appendix

Closes gh-3014

spring-boot-samples/spring-boot-sample-undertow/src/main/resources/application.properties

@@ -1,2 +1,3 @@
 server.undertow.access-log-enabled=true
+server.undertow.access-log-dir=target/logs
 server.undertow.access-log-pattern=combined