Documentation Request - Returned uid from Upsert

[colin-grapl]

How to get assigned UIDs from response

This is a really helpful example for a common use case, but I believe that it doesn't necessarily apply to upserts, or for conditional upserts.

https://dgraph.io/docs/mutations/upsert-block/

I believe that when a node is created as part of an upsert uids will contain the uid, but when the node already exists you have to extract the uid from the json response.

This can be pretty unintuitive and I think it's worth documenting. I don't think I'm necessarily the write person to write that documentation but I'd be happy to help review it.