Missing some chunks of docs

In first place, thank you for fixing the huge amounts of broken links in docs for Qt 5.14.2, that was really annoying. Still there's some chunks that are missing, let's comments the ones I remember. I may update this report with new ones (and appropriate comments) if I found more until this issue is closed.

1. ListModel, ListElement and related functions, probably a regression in Qt 5.14.0 or Qt 5.14.1.
2. qmlRegisterSingletonInstance(), a very useful function that is missing even from its first release.

Thank you in advance.