As a reader and a fan of your work, and as someone who bought your book to learn Elixir, I believe it would be nice to mention this detail.
Yes, it may be completely useless for someone who already knows Supervisors and GenServers and the interactions with init. It may even be useless for someone who simply copy/pastes your code and runs it.
But for someone like me, who genuinely tries to understand the things that are going on and how to piece everything together using dialyzer in order to make (ever so slightly) more robust apps, such a paragraph could help.
It also prevent idiots like me from creating posts like this 
PS: This is just my opinion, you don’t have to agree
