Disable Lumberjack and Log4j 1.x and document it (#182)

This PR removes Lumberjack and Log4j 1.x for the automatic logging
backend discovery procedure.

The updated procedure is documented in the guide.

This closes LOGGING-173 and LOGGING-188.

src/main/java/org/apache/commons/logging/impl/LogFactoryImpl.java

@@ -150,10 +150,8 @@ public class LogFactoryImpl extends LogFactory {
      * but broken/unusable for some reason.
      */
     private static final String[] classesToDiscover = {
-            LOGGING_IMPL_LOG4J_LOGGER,
-            "org.apache.commons.logging.impl.Jdk14Logger",
-            "org.apache.commons.logging.impl.Jdk13LumberjackLogger",
-            "org.apache.commons.logging.impl.SimpleLog"
+            LOGGING_IMPL_JDK14_LOGGER,
+            LOGGING_IMPL_SIMPLE_LOGGER
     };
 
     /**