Docs: Add mentions about Type System

[diggy (Dgraph Bot)]

Moved from GitHub dgraph/4263

Posted by MichelDiz:

Documentation

Ref: How to delete a "row"?

(2) How do I delete using a type? Get started with Dgraph doesn’t even mention type.

[LGalatin (Leyla Galatin)]

Fixed by: docs(Query): Update docs to note that only typed data can be deleted using `S * *`(Docs) by aaroncarey · Pull Request #6657 · dgraph-io/dgraph · GitHub