When I uninstall a version of Qt e.g I remove 5.9.1 and replace it with 5.9.2, the documentation of the 5.9.1 is left on disk and is still registered in Qt Creator.

The most direct observable consequence is that when I want to open the documentation of a class, let's say QString, the Assistant prompts if I want to open the documentation of  "QString Class | Qt Core 5.9" or "QString Class | Qt Core 5.9".

This is not new to Qt 5.9, I had already seen this behavior with previous Qt versions.