Document disabling protocol upgrade for Apache HC5. Fixes gh-1141.

Author: OlgaMaciaszek

docs/modules/ROOT/pages/spring-cloud-openfeign.adoc

@@ -146,7 +146,25 @@ When it comes to the Apache HttpClient 5-backed Feign clients, it's enough to en
 You can customize the HTTP client used by providing a bean of either `org.apache.hc.client5.http.impl.classic.CloseableHttpClient` when using Apache HC5.
 
 You can further customise http clients by setting values in the `spring.cloud.openfeign.httpclient.xxx` properties. The ones prefixed just with `httpclient` will work for all the clients, the ones prefixed with `httpclient.hc5` to Apache HttpClient 5, the ones prefixed with `httpclient.okhttp` to OkHttpClient and the ones prefixed with `httpclient.http2` to Http2Client. You can find a full list of properties you can customise in the appendix.
-If you can not configure Apache HttpClient 5 by using properties, there is an `HttpClientBuilderCustomizer` interface for programmatic configuration.
+If you can not configure Apache HttpClient 5 by using properties, there is an `HttpClient5FeignConfiguration.HttpClientBuilderCustomizer` interface for programmatic configuration.
+
+TIP: Apache HTTP Components `5.4` have changed defaults in the HttpClient relating to HTTP/1.1 TLS upgrades. Most proxy servers handle upgrades without issue, however, you may encounter issues with Envoy or Istio. If you need to restore previous behaviour, you can use `HttpClient5FeignConfiguration.HttpClientBuilderCustomizer` to do it, as shown in the example below.
+
+[source,java,indent=0]
+----
+@Configuration
+public class FooConfiguration {
+
+	@Bean
+	public HttpClient5FeignConfiguration.HttpClientBuilderCustomizer httpClientBuilder() {
+		return (httpClientBuilder) -> {
+			RequestConfig.Builder requestConfigBuilder = RequestConfig.custom();
+			requestConfigBuilder.setProtocolUpgradeEnabled(false);
+			httpClientBuilder.setDefaultRequestConfig(requestConfigBuilder.build());
+		};
+	}
+}
+----
 
 TIP: Starting with Spring Cloud OpenFeign 4, the Feign Apache HttpClient 4 is no longer supported. We suggest using Apache HttpClient 5 instead.