Improve documentation on custom secure transport management

Was: Support TLS-PSK in QMqttConnection

We are using the QtMqtt library for our own MQTT client written in C++ on several desktop platforms. We recently came across a customer requirement to support TLS-PSK because of easier setup compared to certificate-based TLS.

I saw the existing signal QSslSocket::preSharedKeyAuthenticationRequired, but no direct support in your MQTT library. Nevertheless, I managed to implement it on my own by accessing the mentioned signal and providing the broker configuration data.

As it was no big deal, I suggest to integrate the support for TLS-PSK also officially into your library.