documentation of arguments

[stranac]

Well, positional-only arguments are only a thing with built-in functions, as python has no syntax for those (can be emulated using *args though).
I guess you just have to learn which functions and arguments those are, as I can't find them detailed anywhere in the docs.

As for keyword-only args, those are usually shown clearly, at least in python 3, where syntax for something like that exists, check e.g. https://docs.python.org/3/library/functions.html#max

while it only accepts key as a keyword

You can pass a key function as a positional argument to sorted()...