[doc] make clear what happens between nodes

[hartym]

cf slack conversation :

Tomas :
if a function returns bytes, is it possible that the next node will iterate them and receive ints?
If node A returns one list of 20 lists of 5 ints, how does bonobo know whether it should run node B one time, 20 times or 5 times?

Tomas :
I had a function that returned some text. When I returned it as a string. The next function got one string. When I returned it as bytes, the next object got an int. So I was assuming it was iterating over the bytes object, but not the string for some reason.