Documentations of DGLGraph.__getitem__

## 📚 Documentation

Hi, I have some questions about the [docs](https://docs.dgl.ai/en/latest/generated/dgl.DGLGraph.__getitem__.html#dgl-dglgraph-getitem) of `DGLGraph.__getitem__`:

1. One of the most common use cases is that we want to get a subgraph with all edges between `srctype` and `dsttype`, and we will expect the node feature of srctype/dsttype to be shared with the original graph. But there might exist multiple canonical edge types, so according to the doc, the node data won't be shared and will be duplicated.
1. It seem that `dgl.combine_names` used in the doc is not an exposed API.
1. The sentence `The source/destination node would be formed by concatenating the common features of the original source/destination types.` is a little bit confusing. Does it mean the new node features is formed by concatenation?