Clarify CLI usage for CiviMail cron job

This page in the documentation is found easily through a web search https://docs.civicrm.org/sysadmin/en/latest/setup/civimail/#setting-up-the-cron-job

However it was unclear which exactly are the minimum requirements for the mailprocess user. This pull requests adds a link to the place in the docs where I found this information.

I also clean up the -j option which apparently does not exist, at least not in the --help of cli.php. And clarify that -s is optional if it is no multisite install.

By the way, the edit button on https://lab.civicrm.org/-/ide/project/documentation/docs/sysadmin/tree/master/-/docs/setup/civimail/index.md let me use the WebIDE directly, but when I wanted to commit to an automatically generated branch, it would fail with “unknown error” (or similar). I had to manually create a fork first.