[3.14] Touch up `Setup.local` handling in `Tools/wasm/wasi` (GH-137051) (GH-137053)

Touch up `Setup.local` handling in `Tools/wasm/wasi` (GH-137051)

The comment in the generated file is now more self-explanatory. The checks for unexpected file contents are also strengthened.
(cherry picked from commit ec7fad79d24e79961b86e17177a32b32bb340fe5)

Co-authored-by: Brett Cannon <brett@python.org>
Co-authored-by: Zachary Ware <zach@python.org>