Docs are inaccurate for `dag get`

Our docs for the `dag get` command are not entirely accurate and invite some misconceptions: https://github.com/ipfs/go-ipfs/blob/3296412d19d5a7b565ece21f5d7c275c41bcf246/core/commands/dag/dag.go#L186-L192

In particular, it doesn’t really “get a DAG node” so much as it gets the value at an IPLD path. For example:

```sh
> curl 'http://localhost:5001/api/v0/dag/get?arg=zdpuAnhvECCigEM92DCZVwFguAHTSg1yJcPLyPdsJsVTYRuXB/title'
# "A simple example of an IPLD node"
```

I guess you could sort of say that’s technically a node — *kind of* — but I think most people would interpret the node in the above example to be the data at:

```
/ipfs/zdpuAnhvECCigEM92DCZVwFguAHTSg1yJcPLyPdsJsVTYRuXB
```

…not the string at:

```
/ipfs/zdpuAnhvECCigEM92DCZVwFguAHTSg1yJcPLyPdsJsVTYRuXB/title
```

What’s a better way to explain this?

See also this discussion: https://github.com/ipld/ipld/issues/44#issuecomment-411873611. /cc @mikeal @diasdavid @whyrusleeping

Once we figure out the right language, this also needs updating in [`interface-ipfs-core`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/DAG.md#dagget), in js-ipfs’s help text, and in the docs.ipfs.io site (which is generated from the code here).