doc: document fake ENOTFOUND as a system error

PR-URL: https://github.com/nodejs/node/pull/26495
Fixes: https://github.com/nodejs/node/issues/26484


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>