Explain new directory init modes in studio

Following up NXS-5459 , doc needs to be updated.

Goal is to provide a user-centric explanation to what happens depending on the policies chosen and when that happens:

• When is a vocabulary update triggered (e.g. after every nuxeo server restart? When deploying studio config using hot reload?)
• What happens to existing entries
  • that were updated inside Nuxeo Server by a user
  • that were not updated in Nuxeo Server but changed in Studio
• What happens to new entries: are they merged or not?

We need to check with Manon Lumeau first where this would be best explained:

• https://doc.nuxeo.com/studio/vocabularies/ may not be the best option
• https://doc.nuxeo.com/nxdoc/next/data-lists-and-directories/ is focused on the technical implementation and doesn't give a clear answer to the questions above