Add note that in Sentry Log4j2 integration, the async mode is not supported (#4271)

Author: maciejwalkowiak

src/includes/getting-started-config/java.log4j2.mdx

@@ -25,6 +25,8 @@ The `ConsoleAppender` is provided only as an example of a non-Sentry appender se
 </Configuration>
 ```
 
+`SentryAppender` does not support Log4j2's [async mode](https://logging.apache.org/log4j/2.x/manual/async.html). The Sentry Java SDK itself is already asynchronous and does not perform any blocking operation on the calling thread.
+
 ### DSN Configuration
 
 Note that **you need to configure your DSN** (client key) only if you wish to initialize the SDK through the log4j2 integration. If you're planning to use `Sentry.init` to provide configuration, such as by using the `beforeSend` callback, you **should not** provide the DSN in both `Sentry.init` and the appender configuration; just leave it out of the appender configuration in this case.