DOC: Clarify docs for groupby when passed a list with length equal to length of axis
### Documentation location

https://pandas.pydata.org/pandas-docs/dev/reference/api/pandas.DataFrame.groupby.html

### Reproducible Example

```python
import pandas as pd

df = pd.DataFrame([['a', 'b'], ['b', 'c']], columns=['foo', 'bar'])
df.groupby(['y', 'z']).first()

>>>
  foo bar
y   a   b
z   b   c
```

While it is mentioned in the user-guide, I think it would be clearer to include lists (along with the existing mention of the ndarray) to reflect that the values will be used as-is to determine the groups if the lengths match that of the selected axis.
