docs: adjust wording of LEVEL section

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`?

Start with the linked LEVEL section in the Docker Minecraft Server documentation and review its current wording about multiple containers and -v volumes. Done means the volume guidance is unambiguous, the mapping to the level-name property is documented, and the scope of a possible LEVEL_NAME fallback is resolved.

Written by the indexing model from the issue text.