[#3003] 4096 Password length details

Hi guys!

See #3003 - as @fabpot laid out all the details very cleanly there. I believe this covers all the areas that need to be updated, but if anyone spots any issues, please let me know!

Once this is merged into master, I'll change the language in the `authentication` section to talk about using the `checkPasswordLength` that's available in 2.4.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.2+
| Fixed tickets | #3003

Thanks!