bluetooth: Clarify how StartNotifySession function works

device/bluetooth/bluetooth_gatt_characteristic.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef DEVICE_BLUETOOTH_BLUETOOTH_GATT_CHARACTERISTIC_H_
 #define DEVICE_BLUETOOTH_BLUETOOTH_GATT_CHARACTERISTIC_H_
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 163 matching lines @@
   // to implement the delegate methods.
   //
   // This method only makes sense for local characteristics and does nothing and
   // returns false if this instance represents a remote characteristic.
   virtual bool UpdateValue(const std::vector<uint8_t>& value) = 0;
 
   // Starts a notify session for the remote characteristic, if it supports
   // notifications/indications. On success, the characteristic starts sending
   // value notifications and |callback| is called with a session object whose
   // ownership belongs to the caller. |error_callback| is called on errors.
+  //
+  // Writes to the Client Characteristic Configuration descriptor to enable
+  // notifications/indications. Core Bluetooth Specification [V4.2 Vol 3 Part G
+  // Section 3.3.1.1. Characteristic Properties] requires this descriptor to be
+  // present when notifications/indications are supported. If the descriptor is
+  // not present |error_callback| will be run.
   virtual void StartNotifySession(const NotifySessionCallback& callback,
                                   const ErrorCallback& error_callback) = 0;
 
   // Sends a read request to a remote characteristic to read its value.
   // |callback| is called to return the read value on success and
   // |error_callback| is called for failures.
   virtual void ReadRemoteCharacteristic(
       const ValueCallback& callback,
       const ErrorCallback& error_callback) = 0;
 
@@ skipping 11 matching lines @@
   BluetoothGattCharacteristic();
   virtual ~BluetoothGattCharacteristic();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(BluetoothGattCharacteristic);
 };
 
 }  // namespace device
 
 #endif  // DEVICE_BLUETOOTH_BLUETOOTH_GATT_CHARACTERISTIC_H_