Document blocking I/O and threading considerations

- Document that disconnectFromHost() may block while waiting for pending
data to be written, with a timeout of up to 30 seconds.

- Recommend keeping QMqttClient and its transport outside the UI thread
because blocking I/O may occur and affect UI responsiveness.

Pick-to: 6.12
Task-number: QTBUG-150345
Change-Id: I27d300210750710c4a5138f8c3f97c7c929466af
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>