[DOCS] Expand logging documentation

Updated log4j link so it doesn't point to log4j 2.0 but version 1.2. Clarified which formats are supported and briefly explained what loggers and appenders are, plus added a link to the log4j docs.

Closes #5305
Closes #8455

docs/reference/setup/configuration.asciidoc

@@ -307,6 +307,15 @@ All of the index level configuration can be found within each
 === Logging
 
 Elasticsearch uses an internal logging abstraction and comes, out of the
-box, with http://logging.apache.org/log4j/[log4j]. It tries to simplify
+box, with http://logging.apache.org/log4j/1.2/[log4j]. It tries to simplify
 log4j configuration by using http://www.yaml.org/[YAML] to configure it,
-and the logging configuration file is `config/logging.yml` file.
+and the logging configuration file is `config/logging.yml`. The
+http://en.wikipedia.org/wiki/JSON[JSON] and
+http://en.wikipedia.org/wiki/.properties[properties] formats are also
+supported. The logger section contains the java packages and their
+corresponding log level, where it is possible to omit the `org.elasticsearch`
+prefix. The appender section contains the destinations for the logs.
+Extensive information on how to customize logging and all the supported
+appenders can be found on the
+http://logging.apache.org/log4j/1.2/manual.html[log4j documentation].
+