[3.13] gh-142965: Fix Concatenate documentation to reflect valid use cases (GH-143316) (#148900)

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.
(cherry picked from commit 75ff1afcb6a1bb2b3d54899e9b222a61798fa491)

Co-authored-by: John Seong <39040639+sandole@users.noreply.github.com>