Doc: Update processing details topic

/reference/processing.md

Discuss how to handle this topic. What else does it need?
We should remove the "still working on it note" because it's been a long time since the last update.

Start with /reference/processing.md and review its current processing details and the still-working-on-it note. Identify what the topic is missing and confirm the intended scope before proposing documentation changes; done means the topic is updated and the outdated note is removed.

Written by the indexing model from the issue text.