Complete Kotlin example for HTTP message codecs in reference doc

[Attacktive]

It already has one written in Java:

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	@Override
	public void configureHttpMessageCodecs(ServerCodecConfigurer configurer) {
		configurer.defaultCodecs().maxInMemorySize(512 * 1024);
	}
}

It's straightforward to translate to Kotlin so...