Improve QBluetoothSocket::close() documentation

The disconnected() and stateChanged() signals are delayed which
may easily overlooked by API users. This change specifically points out
this behavior difference.

Change-Id: I3a9e9c656da50c9efe455cd2dbf903a2849e36da
Task-number: QTBUG-41770
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>