Add comment explaining protocol for proxy resolver configuration.

tests/integration/test_s3_with_proxy/configs/config.d/storage_conf.xml

@@ -17,6 +17,11 @@
                 <access_key_id>minio</access_key_id>
                 <secret_access_key>minio123</secret_access_key>
                 <proxy>
+                    <!--
+                        At each interaction with S3 resolver sends empty GET / request to specified endpoint to obtain proxy host.
+                        Proxy host is returned as string in response body.
+                        Then S3 client uses proxy URL formed as proxy_scheme://proxy_host:proxy_port to make request.
+                    -->
                     <resolver>
                         <endpoint>http://resolver:8080</endpoint>
                         <proxy_scheme>http</proxy_scheme>