[3.14] ast docs: Fix description of ast.Constant (GH-134741) (#134911)

[3.14] ast docs: Fix description of ast.Constant (GH-134741) (#134911)

Contrary to the current docs, ast.Constant will never hold containers
such as frozenset or tuple; the Python parser only emits it for simple
literals.

For precision, add the exact list of types that may be contained in an
ast.Constant.
(cherry picked from commit 381020d41fb1f8b33421f01c609ba0d0edb99764)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>