Clarify the valid use locations of `typing.Concatenate`

In python/cpython#142965, it was reported that the documentation of typing.Concatenate is "incorrect" (documentation that originates back to python/cpython#24000).

@A5rocks, in your example, Concatenate[int, P_2] somewhat finally lands as a first argument to Callable, just indirectly.
This snippet previously defined:

P_2 = ParamSpec("P_2")

class X(Generic[T, P]):
  f: Callable[P, int]
  x: T

Similar example is included in the typing spec:

https://typing.python.org/en/latest/spec/generics.html#user-defined-generic-classes

class X[T, **P]:
    f: Callable[P, int]
    x: T

# (...)
def accept_concatenate[**P](x: X[int, Concatenate[int, P]]) -> str: ...  # Accepted

However, it seems that the current situation (confirmed with mypy, pyright and ty) is that you can use Concatenate in all valid locations of ParamSpec except directly in a Concatenate. I.e., you can't do Concatenate[int, Concatenate[str, P]]), but you can (besides passing Concatenate form as the first argument to Callable):

1. Accumulate Concatenates as ParamSpecs

from collections.abc import Callable
from typing import Concatenate

type Y[**P] = Callable[Concatenate[int, P], None]
type X[**P] = Y[Concatenate[int, P]]

def foo(f: X[str]) -> None:
    reveal_type(f)
    # mypy: def (builtins.int, builtins.int, builtins.str)
    # pyright: (int, int, str) -> None
    # ty: (...) -> None

2. Bind Concatenates to user-defined generics as ParamSpecs

class X[T, **P]:
    f: Callable[P, int]
    x: T

def accept_concatenate[**P](x: X[int, Concatenate[int, P]]) -> str: ...

3. Use Concatenate as a type argument to tuple -- is this correct?

from typing import Concatenate

def c(t: tuple[Concatenate[int, ...]]) -> None:
    reveal_type(c)
    # mypy: def (t: tuple[[builtins.int, *Any, **Any]])
    # pyright: (t: tuple[Concatenate[int, ...]]) -> None
    # ty: def c(t: tuple[@Todo]) -> None

I've found this mostly by poking around -- I haven't analyzed the actual implementations (yet).
I'll continue to investigate this from these searches.

I think that the valid use locations of Concatenate should be clarified in the typing spec and then in the CPython docs.

Is there anything else I overlooked? CC @JelleZijlstra @AlexWaygood

Start with the typing spec's user-defined generic classes section and the CPython documentation for typing.Concatenate, then compare the examples and the reported behavior from mypy, pyright, and ty. Review the linked Concatenate search notes and determine which use locations are valid; done means the typing spec and CPython docs clearly describe the supported forms and the nested-Concatenate restriction.