Added more doc for HeartbeatService, fixed typo in FileService doc (#875)

Author: artem-v

services-gateway/src/main/java/io/scalecube/services/gateway/websocket/HeartbeatService.java

@@ -4,10 +4,14 @@
 import io.scalecube.services.annotations.ServiceMethod;
 import reactor.core.publisher.Mono;
 
+/**
+ * Service interface for handling custom ping/pong service messages for websocket - service is
+ * echoing back ping message to the client. Used (optionally) as part of {@link WebsocketGateway}.
+ */
 @Service(HeartbeatService.NAMESPACE)
 public interface HeartbeatService {
 
-  String NAMESPACE = "v1/scalecube.websocket.heartbeat";
+  String NAMESPACE = "v1/scalecube.websocket";
 
   @ServiceMethod
   Mono<Long> ping(long value);

services/src/main/java/io/scalecube/services/files/FileService.java

@@ -5,7 +5,7 @@
 import reactor.core.publisher.Mono;
 
 /**
- * System service interface for adding files locally, those added files will be accessible by {@link
+ * Service interface for adding files locally, those added files will be accessible by {@link
  * FileStreamer}. Typical usage: client generates a file in the app service, then calls {@link
  * #addFile(AddFileRequest)}, then returns result (file path qualifier) all the way back to the
  * caller of app service. On the caller side file path qualifier gets combined with http-gateway