Clarify documentation for Jellyfin

[QEDeD]

Currently the documentation doesn't call out that the in the default config suggestion should be configured.

It also isn't clear if the playbook does anything with regards to handling creation of the directories, should the directories not exist on the managed node.

jellyfin_container_additional_volumes:
  - type: bind
    src: /path/on/the/host/movies
    dst: /movies

  - type: bind
    src: /another-path/on/the/host/anime
    dst: /anime
    options: readonly

https://github.com/mother-of-all-self-hosting/mash-playbook/blob/main/docs/services/jellyfin.md