https://github.com/mongodb/mongo-go-driver/blob/d685dac01f10a738bb94f874071281022cddc55d/x/network/wiremessage/msg.go#L174

The main document isn't always in the first document. For instance, running delete in the shell puts the main document (seemingly) last. I think we just need to loop until you find a type 0 section and use that.