Clarifying raw_mimetype?

Hi,

When experimenting in a Jupyterlab instance, if I change the "Raw NBConvert Format", it results in for example

```json
{
"cell_type": "raw",
"metadata": {
"raw_mimetype": "text/restructuredtext",
"tags": []
},
"source": []
}
```

but from [the doc here](https://nbformat.readthedocs.io/en/latest/format_description.html#raw-nbconvert-cells):

```json
{
"cell_type" : "raw",
"metadata" : {
# the mime-type of the target nbconvert format.
# nbconvert to formats other than this will exclude this cell.
"format" : "mime/type"
},
"source" : "[some nbformat output text]"
}
```

The difference is the former uses `raw_mimetype` as the key of the mime-type, and later uses `format`.

Could you clarify the usage of these 2? Thanks!