Document Android's caching of BTLE service data.

It can be confusing. As more platforms are supported, this behavior
needs to be verified too. The Bluez implementation does not exhibit this
cache behavior.

Change-Id: I42dd003f2ac2632c32e48b1b1784e4ca529950f3
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>
QLowEnergyService::writeCharacteristic - implementation for OS X and iOS

writeCharacteristic - implemented with Core Bluetooth for OS X and iOS.

Change-Id: Ia228ff451e1e6d7b6fb7de6cad29198aa9257602
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>