Provide better guidance about Apify.main()

[jancurn]

Seems quite a lot of people end up copy-pasting code and using Apify.main() in situations where it shouldn't be used. We should probably improve the docs somehow or perhaps even get rid of the function altogether, and replace it with e.g. Apify.setup(). Dunno...

https://stackoverflow.com/questions/62357401/express-stops-listening-when-using-puppeteer
https://stackoverflow.com/questions/58543882/is-there-a-way-to-use-apify-main-without-it-exiting-the-node-js-process-on-com
https://stackoverflow.com/questions/56977763/how-to-use-apify-on-google-cloud-functions