Drop paragarph about anonymous (unnamed) volumes under Persistent Storage section fo the docs

[joshtrichards]

The opening paragraphs for Persistent Storage in the README are somewhat misleading unless you already know what anonymous (unnamed) volumes are and their implications. It almost sounds like you don't need to do have a named volume at all, but then the next paragraph goes on to suggest otherwise without further context. We can probably tighten this section up a bit.

Current:

The Nextcloud installation and all data beyond what lives in the database (file uploads, etc.) are stored in the unnamed docker volume volume /var/www/html. The docker daemon will store that data within the docker directory /var/lib/docker/volumes/.... That means your data is saved even if the container crashes, is stopped or deleted.

A named Docker volume or a mounted host directory should be used for upgrades and backups. To achieve this, you need one volume for your database container and one for Nextcloud.

Currently I'm leaning towards dropping the first paragraph entirely, since it's more a behind-the-scenes implementation detail and not one particularly relevant to admins. There aren't any realistic scenarios where relying on the anonymous volume is a valid/supported configuration (outside the lab maybe) that I can think of.