Document isLog4jAutoShutdownDisabled to enable logging during a web application listener's contextDestroyed

As discussed in issue [LOG4J2-1606](https://issues.apache.org/jira/browse/LOG4J2-1606), a flag was added to address the problem of log4j-web shutting down Log4j when other listeners defined in the web app still want to log messages during their contextDestroyed. However, this is not documented anywhere and it was quite difficult for me to find the solution. I suggest documenting this in [Using Log4j 2 in Web Applications](https://logging.apache.org/log4j/2.x/manual/webapp.html) or [Frequently Asked Questions](https://logging.apache.org/log4j/2.12.x/faq.html):

When your web application includes log4j-web (as it [should](https://logging.apache.org/log4j/2.x/manual/webapp.html)) and you would like to use Log4j to log messages when one of your listener's `contextDestroyed` method is called, include the following near the top of your `web.xml`:

```xml

isLog4jAutoShutdownDisabled
true

org.apache.logging.log4j.web.Log4jShutdownOnContextDestroyedListener

```

Note that for Servlet 3.x, you don't need to include your listener in your `web.xml` if it is annotated with `@WebListener`. The above solution will still work for you.

Start with the “Using Log4j 2 in Web Applications” page and the referenced Frequently Asked Questions page. Document the isLog4jAutoShutdownDisabled context parameter, the Log4jShutdownOnContextDestroyedListener registration order, and the Servlet 3.x @WebListener case; done means users can find and follow the web.xml example to keep logging available during contextDestroyed.

Written by the indexing model from the issue text.