docs: clarify how Node.js version is selected

[smaeda-ks]

The underlying package @vercel/build-utils would select the newest available Node.js version unless its specified:

• https://github.com/vercel/vercel/blob/main/packages/build-utils/src/fs/node-version.ts
• https://github.com/vercel/vercel/blob/23dd29e2691823ffbe1b7077cc79f53a47715c1b/packages/build-utils/src/fs/run-user-scripts.ts#L247-L280