I’m no expert and have never given myself over as one.
However, from what I can see from the information you’ve provided, it does make sense seeing as there is only qt5-tools to work with form the -docs packages.
AFAIK, qt4/qt5/qt6 are different versions of the framework, and just as qt5 tools will not work for qt4 and vice versa, qt5/qt6 tools will not work with each other.
So give installing qt6-tools a go and see what happens:
qt6-tools has been installed, but for Qt Assistant, it shows Qt Assistant (qt5-tools) 5.15.2+kde+r17-3.
Also no standalone package for qt5-tools, I think Qt Asstistant and qt5-tools are bundled together.
After some searching on the web, there is qt6-assistant, but no corresponding one in manjaro official repo or aur repo.