Add essential clarification to help with cron debugging

[gwillem]

Knowing about these issues beforehand would probably have saved me a day of debugging.

Purpose of this pull request

This pull request (PR) adds missing architecture details that are required to properly debug cron job problems.

Affected DevDocs pages

• https://devdocs.magento.com/cloud/configure/setup-cron-jobs.html

[dobooth]

@magento import code to magento-devdocs/devdocs

[m2-github-services]

@dobooth the branch with the code is successfully imported into the magento-devdocs/devdocs repository. Branch name: imported-gwillem-devdocs-9515.