IntegerArray docs follow-up
xref #21160

Make it clear that it is opt-in only, by passing/casting dtype.  For example `[24]` and `[27]` are still float64

More clear separation from numpy dtype - worry that `'int64'` vs `'Int64'` will be especially confusing for new people?  Consider a different name altogether?  (`NullableInt64`?)

```python

In [23]: s = pd.Series([1, 2, np.nan])

In [24]: s
Out[24]: 
0    1.0
1    2.0
2    NaN
dtype: float64

In [25]: s = pd.Series([1, 2, 3])

In [26]: s[2] = np.nan

In [27]: s
Out[27]: 
0    1.0
1    2.0
2    NaN
dtype: float64
```