ohos: document the HarmonyOS bluetooth backend

Add HarmonyOS to the supported-platform table and to the logging
category table, so the module documentation names the backend the
preceding commits implement. Classic Bluetooth only: there is no LE
backend on this platform.

Record the deviations an application can observe, next to the API each
one constrains: the system dialog a power state change raises and the
absence of a discoverable timeout; the null local adapter address; the
single entry allDevices() reports, which is gated on the permission and
on the adapter being on, so an empty list does not mean there is no
adapter; the empty connectedDevices() list and the deviceConnected() and
deviceDisconnected() signals that follow from it; the impossibility of
unpairing; the service registration listen() leaves to
QBluetoothServiceInfo; the service discovery that confirms no RFCOMM
service, and synthesises the filtered UUIDs for a single device; the
socket endpoint the platform never exposes; and the remote disconnect a
socket cannot observe.

Record that this backend implements RFCOMM sockets only, beside the
platforms which support nothing else, and state that
connectToService() with a port reports UnsupportedProtocolError rather
than the ServiceNotFoundError the platforms named beside it report,
because this backend rejects the overload outright.

Widen the errorOccurred() description from pairing to host mode
changes as well, because this backend reports a refused or unanswered
bluetooth switch through the same signal.

Task-number: QTBUG-147192
Pick-to: 6.12
Change-Id: Iab9a401ba29fb1ceb221d60ab93e56a2723f49dc
Reviewed-by: Shane Liu <shane.liu@qt.io>
Reviewed-by: Zbigniew Chyla <zbigniew.chyla@qt.io>
Reviewed-by: Liang Qi <liang.qi@qt.io>