Polishing documentation

See gh-608

spring-graphql-docs/modules/ROOT/pages/client.adoc

@@ -165,14 +165,20 @@ existing `WebSocketGraphQlClient` to create a new instance with customized setti
 
 ----
 
-If you'd like the client to send regular graphql ping messages to the server, you can add these by adding `keepalive(long seconds)` to the builder
+`WebSocketGraphQlClient` supports sending periodic ping messages to keep the connection
+active when no other messages are sent or received. You can enable that as follows:
+
 [source,java,indent=0,subs="verbatim,quotes"]
 ----
-    WebSocketGraphQlClient graphQlClient = WebSocketGraphQlClient.builder(url, client)
-        .keepalive(30)
-        .build();
+	URI url = ... ;
+	WebSocketClient client = ... ;
+
+	WebSocketGraphQlClient graphQlClient = WebSocketGraphQlClient.builder(url, client)
+			.keepAlive(Duration.ofSeconds(30))
+			.build();
 ----
 
+
 [[client.websocketgraphqlclient.interceptor]]
 ==== Interceptor