Updated milling documentation

[PatrikLundell]

Summary

None

Purpose of change

Fix #75293, i.e. obsolete documentation for milling.

Describe the solution

Change the only place found where "milling" is referenced to describe the new syntax and reliance on a recipe.

Describe alternatives you've considered

Expand the description. However, the format doesn't lend itself for exhaustive documentation.

Testing

I don't think you can test documentation. I guess there might be auto tests that checks that it adheres to whatever the syntax for the ".md" format is.

Additional context