DOC: Order of groups in groupby and head method
When `sort = True` is passed to `groupby` (which is by default) the groups will be in sorted order. If you loop through them they are in sorted order, if you compute the mean, std... they are in sorted order but if you use the method `head` they are NOT in sorted order.
```

import pandas as pd

df = pd.DataFrame([[2, 100], [2, 200], [2, 300], [1, 400], [1, 500], [1, 600]], columns = ['A', 'B'])

grouped = df.groupby(df['A'], sort = True)
for name, group in grouped:
    print(group)
print(grouped.mean())
print(grouped.head(1))
```
Is this expected? I have not found this behaviour documented.
I think it is confusing and has caused me a headache because I was combining output of the `mean` and `head` methods in the same DataFrame, and since the data was not ordered before those results were getting mixed because of these order issues. I have pandas 0.20.3



