gh-142965: Fix Concatenate documentation to reflect valid use cases

The documentation previously stated that Concatenate is only valid
when used as the first argument to Callable, but according to PEP 612,
it can also be used when instantiating user-defined generic classes
with ParamSpec parameters.

Author: sandole

Doc/library/typing.rst

@@ -1174,7 +1174,8 @@ These can be used as types in annotations. They all support subscription using
    or transforms parameters of another
    callable.  Usage is in the form
    ``Concatenate[Arg1Type, Arg2Type, ..., ParamSpecVariable]``. ``Concatenate``
-   is currently only valid when used as the first argument to a :ref:`Callable <annotating-callables>`.
+   is valid when used in :ref:`Callable <annotating-callables>` type hints
+   and when instantiating user-defined generic classes with :class:`ParamSpec` parameters.
    The last parameter to ``Concatenate`` must be a :class:`ParamSpec` or
    ellipsis (``...``).