Document certificate handling in QtWebEngine

Qt WebEngine differs from Qt WebKit in that it uses it's own network stack. This is also visible in the way certificates are handled.

• QSslConfiguration is not used
• WebEngine uses the root certificates from the OS
• Errors due to invalid certificates can be handled by WebEngineView::certificateError / QWebEnginePage::certificateError

I guess we need to extend this in the future ... anyhow, for the moment documenting the current state (and also mentioning it in the porting guidelines) is a step forward.