docs: COPY DEST/SRC usage with plugins/mods mount points is misleading

[itzg]

Enhancement Type

Improve an existing feature

Describe the enhancement

Discussed in Discord

https://docker-minecraft-server.readthedocs.io/en/latest/mods-and-plugins/#optional-plugins-mods-and-config-attach-points:~:text=The%20source%20can%20be%20changed%20by%20setting%20COPY_PLUGINS_SRC.%20The%20destination%20can%20be%20changed%20by%20setting%20COPY_PLUGINS_DEST.

It should be clarified that the COPY DEST/SRC variables should only be used for very customized scenarios where the defaults paths are not appropriate for some reason.

(Need to investigate why those options even exist and if they should simply be "hidden" for advanced use only)