I would argue that the documentation is the wrong thing here. Really this feature is not as helpful as you think it is! (There is a very high likelihood that you will overwrite the automatic decision later in the design process.)
The downside of allowing this feature to be used is that one must rely on invisible power input pins that automatically connect to a specific power label. This is both unintuitive and inflexible as it does not support having multiple power supply lines in the same design.
But yes sadly there are still libraries that have not yet been converted. The 74xx_IEEE version is just one example.
By the way i handle multi unit symbols with separate power unit in Tutorial: Introduction to PCB design with KiCad version 5.1 (Getting Started)