Add java doc comment for method

components/application-mgt/io.entgra.device.mgt.core.application.mgt.core/src/main/java/io/entgra/device/mgt/core/application/mgt/core/impl/SubscriptionManagerImpl.java

@@ -707,6 +707,13 @@ public class SubscriptionManagerImpl implements SubscriptionManager {
         }
     }
 
+    /**
+     * Validates if all devices in the subscription have pending operations for the application.
+     *
+     * @param devices List of {@link Device}
+     * @param subscribingDeviceIdHolder Subscribing device id holder.
+     * @throws BadRequestException if incompatible data is found with validate pending app operations request
+     */
     private <T> void validatePendingAppSubscription(List<Device> devices, SubscribingDeviceIdHolder subscribingDeviceIdHolder) throws BadRequestException{
         Set<DeviceIdentifier> deviceIdentifiers = devices.stream()
             .map(device -> new DeviceIdentifier(device.getDeviceIdentifier(), device.getType()))