Improve the documentation - Mark as Important

Is your feature request related to a problem? Please describe.

Users do not know what the "Mark as Important" assistance feature does, the side-affects in external email clients, how it does it, and what data is used.

Describe the solution you'd like

Provide documentation to explain:

1. What the feature will do to emails within Nextcloud, including side-affects if those emails are using IMAP storage i.e. it sets the Important tag as seen in Thunderbird.

2. How the feature actually works. By viewing other issues, I understand it uses machine learning of the last 1000 emails, but no less than 20. I think users need the following:

• What is this AI's definition of "Important"?
• How often does ML training occur?
• Which emails are trained on, including all conditional filters the system uses e.g. last 1000 emails from the inbox
• Which email data is used to train on? e.g. subject, body, other attributes?
• How can users correct/reverse the system? Currently, when the tag is reset (manually, or automatically from email clients or APIs), it is set again on the next inference run under unknown circumstances.

Describe alternatives you've considered

No known alternatives are possible within Nextcloud. I rely on external functionality to automate tagging e.g. Rules and add-ons within Thunderbird and the IMAP server.
As this AI feature is currently undocumented, my solution is to not use it at all.

Additional context

This issue is as a result of noticing ALL inbox emails within a certain account in Thunderbird being marked as Important. After reviewing all rules in all systems that I control, I confirmed the culprit feature within the Nextcloud Mail app by searching through issues. Unfortunately, I installed the Mail app before the fix #9609 was implemented to change the setting default to "inactive".

I'm comfortable with software configuration so I could solve my problem, but I guess less knowledgeable users might uninstall the app, or even live with the inconvenience assuming they don't realise the feature is their problem. This ticket could prevent both situations.
Thanks.

Start by reviewing the Mark as Important feature and the related issue referenced in the report, then verify its documented behavior with IMAP clients such as Thunderbird. Done means explaining the feature's effects, training inputs and conditions, training frequency, definition of importance, and how users can correct or reverse its results.