BUG/QST: Series.transform with a dictionary
What is the expected output of passing a dictionary to `Series.transform`? For example:

    s = pd.Series([1, 2, 3])
    result1 = s.transform({'a': lambda x: x + 1})
    result2 = s.transform({'a': lambda x: x + 1, 'b': lambda x: x + 2})

The docs say that `dict of axis labels -> functions` is acceptable, but I can't find any example in the docs where the output is described/shown. Under the hood, `Series.transform` is just calling `Series.aggregate` which produces the following outputs for `result1` and `result2`.

````
# result1
a  0    2
   1    3
   2    4
dtype: int64

# result2
a  0    2
   1    3
   2    4
b  0    3
   1    4
   2    5
dtype: int64
````

`result1` is deemed acceptable (the length of the result equals the length of the input) and is returned, but `result2` raises; it is not a transformation.

I am wondering if a better return would be a DataFrame where the keys are the column names ('a' and 'b' in this example).