doc: warn users about non-ASCII paths on build

The build breaks if there's a non-ASCII character on
the path to the building directory.

PR-URL: https://github.com/nodejs/node/pull/16735
Refs: https://github.com/nodejs/node/issues/16278
Refs: https://github.com/nodejs/node/issues/14336


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>