Document `*.pyw` is included by default in preview (#21885)
Document `*.pyw` is included by default in preview mode.
Originally requested in https://github.com/astral-sh/ruff/issues/13246
and added in https://github.com/astral-sh/ruff/pull/20458
Co-authored-by: Amethyst Reese <amethyst@n7.gg>
