document groupby(mutated=True)
#### Code Sample, a copy-pastable example if possible

```python
(Pdb) df = DataFrame(range(10), columns=['a'])
(Pdb) df
   a
0  0
1  1
2  2
3  3
4  4
5  5
6  6
7  7
8  8
9  9

(Pdb) df.groupby(df.a + df.a % 2).apply(lambda x : x)
   a
0  0
1  1
2  2
3  3
4  4
5  5
6  6
7  7
8  8
9  9

(Pdb) df.groupby(df.a + df.a % 2).apply(lambda x : "foo")
a
0     foo
2     foo
4     foo
6     foo
8     foo
10    foo

(Pdb) df.groupby(df.a + df.a % 2, mutated=True).apply(lambda x : x)
      a
a      
0  0  0
2  1  1
   2  2
4  3  3
   4  4
6  5  5
   6  6
8  7  7
   8  8
10 9  9

```
#### Problem description

`GroupBy.apply` is not consistent about whether or not it adds group keys to the index.   Sometimes it adds them, sometimes not, based on what it thinks is the right thing to do.

If you never want them added you can pass `group_keys=False`, but if you always want them added you must pass `mutated=True`, which is not documented.


