docs(api): update description for WEBPACK_SERVE env variable

[snitin315]

Update description to include server command/alias as well.

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/webpack-docs/webpack-js-org/99x1FUu6WRUVYGddGwqrW1cLLimJ
✅ Preview: https://webpack-js-org-git-update-cli-webpack-serve-webpack-docs.vercel.app