Document requirement for non-rt twin of each rt data structure

Essence:

• You need to loop over RT data structure in RT loop
• You want to loop over non RT data structure in non RT loop (processing of package sent from RT to non RT)
• typeinfo() is required on non RT side, but disabled/non-accessible on RT side (due to -fno-rtti, see)
• For example, for sensors, the non RT part does not necessarily have to allocate memory for the package from RT part, because a higher instance (e.g., unit) can query the required amount of memory via the serializer interface

@dreher @kartmann