doc: improve explanation of package.json "type" field

Regardless of the value of the `"type"` field, `.mjs` files are
always treated as ES modules and `.cjs` files are always treated
as CommonJS.

PR-URL: https://github.com/nodejs/node/pull/27516


Reviewed-By: Bradley Farias <bradley.meck@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>