2448: Give a chance to rspamd's bayes classifier r=mergify[bot] a=nextgens
## What type of PR?
bug-fix + documentation
## What does this PR do?
As pointed out in #2442, the bayesian filter of rspamd doesn't get any chance to run as ``min_learns`` is set to 200 and we never teach it any HAM.
This PR enables rspamd's autolearn feature, that will "reinforce" good/bad by learning from the scoring of other modules. It ensures both that we will eventually reach the 200 mark but also that the data stays fresh.
I've also taken this opportunity to update the documentation & FAQ accordingly, to ensure that users teach their HAM & SPAM to both the fuzzy and bayes classifiers.
Thank you to [woj-tek](https://github.com/woj-tek) for doing the ground work on this.
### Related issue(s)
- closes#2442
## Prerequisites
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.
- [x] In case of feature or enhancement: documentation updated accordingly
- [x] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/workflow.html#changelog) entry file.
Co-authored-by: Florent Daigniere <nextgens@freenetproject.org>
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