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-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ServerProperties.java

@@ -582,7 +582,7 @@ public class ServerProperties implements EmbeddedServletContainerCustomizer, Ord
 		/**
 		 * Format pattern for access logs.
 		 */
-		private String accessLogPattern;
+		private String accessLogPattern = "common";
 
 		/**
 		 * Enable access log.
@@ -590,9 +590,9 @@ public class ServerProperties implements EmbeddedServletContainerCustomizer, Ord
 		private boolean accessLogEnabled = false;
 
 		/**
-		 * Undertow access log directory. If not specified a temporary directory will be used.
+		 * Undertow access log directory.
 		 */
-		private File accessLogDir;
+		private File accessLogDir = new File("logs");
 
 		public Integer getBufferSize() {
 			return this.bufferSize;
@@ -635,7 +635,7 @@ public class ServerProperties implements EmbeddedServletContainerCustomizer, Ord
 		}
 
 		public String getAccessLogPattern() {
-			return accessLogPattern;
+			return this.accessLogPattern;
 		}
 
 		public void setAccessLogPattern(String accessLogPattern) {
@@ -643,7 +643,7 @@ public class ServerProperties implements EmbeddedServletContainerCustomizer, Ord
 		}
 
 		public boolean isAccessLogEnabled() {
-			return accessLogEnabled;
+			return this.accessLogEnabled;
 		}
 
 		public void setAccessLogEnabled(boolean accessLogEnabled) {
@@ -651,7 +651,7 @@ public class ServerProperties implements EmbeddedServletContainerCustomizer, Ord
 		}
 
 		public File getAccessLogDir() {
-			return accessLogDir;
+			return this.accessLogDir;
 		}
 
 		public void setAccessLogDir(File accessLogDir) {