Document backup/restore behavior with Scylla API

[Michal-Leszczynski]

In theory those changes should be transparent to the user.

On the other hand, using Scylla API for backup and restore changes some things.
E.g., SM backup/restore flags like --transfers --rate-limit --unpin-agent-cpu don't make any sense when using Scylla API.
On the other hand, user can control backup/restore speed by configuring scylla.yaml to their needs.

In order to avoid confusions, we should explain it in the docs, and perhaps forbid (or at least warn in the logs) from setting those flags when Scylla API is used.

cc: @karol-kokoszka @VAveryanov8 @regevran

[regevran]

I agree.
If we use rclone then rclone flags should be available.
If we are not using it then its flag shouldn't be available either in the form of an error or a warning.