Document private methods

[KR155E]

Apparently Doxygen only catches methods which are listed in the class definition header files. Thus, it's missing private methods and go-to-definition therefore is not working for those.

To solve that we'd have to add those methods to the header files, and decorate the section with /// @privatesection.

Since that would make them public, though, we need to rely on a workaround. Which is to add only comment blocks for the private methods and annotate which methods these are for.

An example can be found in https://github.com/VUEngine/VUEngine-Plugins/blob/f040a45f7050b687753516263df5e35790d71be6/other/LowPowerIndicator/source/LowPowerManager/LowPowerManager.h#L71