Update documentation for NonDeletingInverseHomomorphism

[akoehn]

1. The documentation states:

 * This automaton has the same states as the base automaton, converted into
 * strings.

But according to my understanding of the code, the states are used directly and not a string interpretation.

2. InverseHomomorphism has a description of what an inverse homomorphism is. This description might be helpful in the NDIH documentation. As there is no inheritance relation between the both, either copying (and adapting) parts of the documentation or explicitly linking might be good.

Assigning @alexanderkoller as you probably wrote the current documentation and might be able to reconstruct what it was supposed to mean.