Where [0] talks about customising the CA certificate database (quite near the end of the section), it thrice refers to use of setDefaultCaCertificates(), which is now deprecated [1]. This leads to the references not being links (making it hard to follow up on how to customise). The documentation here should be updated to replace these references, in the manner indicated in the first line of [1].

[0] http://doc.qt.io/qt-5.5/qsslsocket.html
[1] http://doc.qt.io/qt-5.5/qsslsocket-obsolete.html#setDefaultCaCertificates