Update documentation of Qt D-Bus XML compiler (qdbusxml2cpp)

The documentation https://doc.qt.io/qt-6/qdbusxml2cpp.html actively recommends against running the tool as part of the build process.

However, this recommendation doesn't apply to the common case of generating C++ interfaces, which certainly makes sense as part of the buildsystem. Even generating adaptors makes sense as part of the buildsystem.

On the other hand, there is no need to edit the adaptor by hand afterwards, the documentation should not talk about hand-editing at all.

See discussion: https://codereview.qt-project.org/c/qt/qtbase/+/421653/comments/c68173bd_542f18c8