MQTT `list` may provide paths that are absent at runtime

[ryan-summers]

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If a setting is an Option, and thus run-time dependent, it will still be provided in the list output of the MQTT client.

This could be acceptable, but we should document it. Otherwise, we should update list to use the current settings structure to traverse the present paths instead of all possible paths.

[jordens]

I think it's ok to include absent paths. But it should indeed be documented.