Add class documentation for QLowEnergyService

This class is part of the new Bluetooth Low Energy feature in Qt 5.4

Change-Id: If10cac1ac1312cb63137c854a4456bf2d646f630
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Update QLowEnergyCharacteristic class documentation.

Change-Id: I9f8a2cc1b833e91f05434e68ad3b5b4757eac786
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Update QLowEnergyDescriptor class documentation.

Change-Id: I65d28f43862e95258c3115b79de839b884966119
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>