clarify Bluetooth power situation in docs
clarify Bluetooth power situation in docs

add a note to docs that powering Bluetooth on and off might behave differently on different platforms

Change-Id: I461aa651c21e07bcd56a7c7e7eca9c73084d81d7
Reviewed-by: Alex <3f9b327efd0f2b2e684d51660e829d1496ae5581@nokia.com>

kobolabs/qtconnectivity,kobolabs/qtconnectivity,kobolabs/qtconnectivity