[3.13] gh-149267: Document ast.Constant.kind attribute (GH-149268) (#149294)

gh-149267: Document ast.Constant.kind attribute (GH-149268)

The kind attribute of ast.Constant was not mentioned in the
documentation. It is set to 'u' for u-prefixed string literals
and None for all other constants.

---------
(cherry picked from commit 3a1df787e1e630d3d57e99226604ddceb8c47229)

Co-authored-by: Anuj Nitin Bharambe <119653366+anujbharambe@users.noreply.github.com>
Co-authored-by: Anuj Bharambe <anujnitinb@gmail.com>