In this paper Theorem 3.1 is a coherence theorem for monoidal categories in the string diagram calculus.
I don't mind taking this theorem for granted for now, but I'm not quite sure whether I really understand it: the statement sounds too easy, so much that I'm quite unsure if I'd be using it right.
The Caveat 3.2. following the theorem confuses me even more.
Where can I find a more comprehensive / rigorous explanation of the coherence theorem for monoidal categories using string diagrams?