doc: Windows building supported on x64

Since we are CI testing on x64 hosts, we can only commit to x64 hosts.

PR-URL: https://github.com/nodejs/node/pull/21443
Refs: https://github.com/nodejs/node/issues/21402
Refs: https://github.com/nodejs/node/pull/21437


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matheus Marchini <matheus@sthima.com>
Reviewed-By: João Reis <reis@janeasystems.com>