docs: adjust wording of LEVEL section

[itzg]

https://docker-minecraft-server.readthedocs.io/en/latest/configuration/server-properties/#level-world-save-name

The multiple containers and -v guidance is a bit confusing. Really should just say: make sure two or more containers are not using the same data volume...or maybe not even get into all that.

Also mention that it maps to level-name property.

Maybe the variable should be called LEVEL_NAME with a fallback to LEVEL?

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[itzg]

Keep this alive

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This issue is stale because it has been open 30 days with no activity. Please add a comment describing the reason to keep this issue open.