FWIW here's my feedback (as a Python teacher and doc guy):

I find that newcomers are good to ignore what they don't understand, so a newcomer exposed to "foo(a, b, /)" will no run away nor cry, he will just ignore the slash and understand that "foo takes two parameters, a and b" and be happy with it.

Then I think that for more advanced people it's nice to have it:

- It's a way to discover it's a valid syntax
- It's a usefull information to use
- It does not take a lot of space
- It's the truth (I mean, displaying "foo(a, b)" for "foo(a, b, /)" is a kind of a lie, I don't like it)

So I'm +1 for using PEP570 syntax in the documentation.