Add QBLuetoothDeviceDiscoveryAgent::UnsupportedPlatformError

iOS does not support classic Bluetooth device discovery. However since
iOS 5.0 it is possible to search for Bluetooth Low Energy devices. On
pre 5.0 devices tLE scan is not possible and this error enum is going
to be used.

In addition the enum is utilized by the default backend
which is used on unsupported Bluetooth platforms.

[ChangeLog][QtBluetooth][QBluetoothDeviceDiscoveryAgent] A new
UnsupportedPlatformError enum value added.

Change-Id: I74b85782ba1e8ac55798171151b364d5dc0aa79f
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Further improve the DeviceDiscoveryAgent class documentation.

Change-Id: I405b830a8c6199bef641329096de10cad14894fb
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Fix qdoc warnings & adds missing changelog entry for recent API change

1.) Duplicated class documentation in service discovery agent code

2.) Added missing reference to "\a uuid" parameter in new
QBluetoothDeviceInfo::setDeviceUuid()

[ChangeLog][QtBluetooth][QBluetoothDeviceInfo] New deviceUuid attribute
added to QBluetoothDeviceInfo

Change-Id: I7f87d7b95e28b435cb646d284eb447dbdccf0c90
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>