minor: doxygenize health_care_service_interface.h comments

include/grpc++/health_check_service_interface.h

@@ -47,7 +47,7 @@ class HealthCheckServiceInterface {
  public:
   virtual ~HealthCheckServiceInterface() {}
 
-  /// Set or change the serving status of the given service_name.
+  /// Set or change the serving status of the given \a service_name.
   virtual void SetServingStatus(const grpc::string& service_name,
                                 bool serving) = 0;
   /// Apply to all registered service names.