fix description in traits.rs

[tistatos]

Objective

Missed to update the documentation in traits.rs in #730

Feedback wanted

For my own understanding and for others clarification, I also added the reason why this example is limited to TraversePartialTree - Hopefully it's correctly described!

[nicoburns]

This is not wrong, but I don't think it will make sense to people reading that part of the docs (who may well not know what TraversePartialTree is). Perhaps this would make more sense as a doc comment in the example itself?

[tistatos]

That is a valid point! I will add it to the example. Would you like me to add it at the head of the file? Or in connection with the impl TraversePartialTree ?

[nicoburns]

I think at the head of the file would make most sense.