Placing the comma after the docstring introduces a syntax ambiguity:
def foo(
param: str = "some default value"
"""Some documentation""",
):
...
Is that a string concatenation? Or is the second one a docstring? With today’s syntax, it’s a string concatenation, so if it’s now a docstring then that’s a backwards-incompatible syntax change.
On the other hand, I would agree with @BrenBarn that separating the parameter from its docstring with a comma would be egregious.