API: alignment behaviour of Series as assignment value in indexing
Follow up on https://github.com/pandas-dev/pandas/issues/37427

Currently, there is an inconsistency between `__getitem__` and `loc` related to how the RHS value in an assignment gets re-aligned or not.

In this example, the values in Series get just assigned as is, ignoring the index of the series (only requiring that the length matches the number of values in the indexer):

```python
s = pd.Series(range(5))
idx = np.array([1, 4]) 

>>> s[idx] = Series([10, 11])   
>>> s   
0     0
1    10
2     2
3     3
4    11
dtype: int64
```

However, with `loc` we have a different behaviour: the Series gets basically aligned with `s` first, and then this realigned series is also indexed with ``idx``, and those values get set:

```python
s = pd.Series(range(5))
idx = np.array([1, 4]) 

>>> s.loc[idx] = Series([10, 11]) 
>>> s 
0     0.0
1    11.0
2     2.0
3     3.0
4     NaN
dtype: float64
```

you could write what basically happens more explicitly as:

```python
s.loc[idx] = Series([10, 11]).reindex(s.index)[idx] 
```

To have the same behaviour as `__getitem__`, you need to assign with an array instead of series:

```python
s = pd.Series(range(5))
idx = np.array([1, 4]) 

>>> s.loc[idx] = Series([10, 11]).values    
>>> s 
0     0
1    10
2     2
3     3
4    11
dtype: int64
```