There should be a section in DOCS explaining on how to reconstruct the metadata of the config servers.

If one looses all config servers it should be possible to reconstruct the metadata / config servers since the shards themselves know which chunks they hold. This is probably a pretty trivial task but it is not documented.
It probably could even be automated in a command like repairdb.

If one looses one out of three config servers it should be documented if one just needs to rsync the data of the healthy config servers to a new 3rd one or if any other actions need to be taken so that metadata can recover from readonly mode.
This could probably also be automated in a simple repair command.