Maybe wrong default axis with operators (add, sub, mul, div) between datetime-indexed df and series 1.0.0
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

index = pd.date_range(start='2020', periods=5)
df = pd.DataFrame([[1, 2, 3]] * 5, columns=['a', 'b', 'c'], index=index)
series = pd.Series([10, 20, 30, 40, 50], index=index)

print(df + series)
```

|  | 2020-01-01  00:00:00 | 	2020-01 02 00:00:00 |	2020-01-03 00:00:00 |	2020-01-04 00:00:00	| 2020-01-05 00:00:00	| a |	b |	c |
|---|---|---|---|---|---|---|---|---|
| 2020-01-01 |	NaN |	NaN |	NaN |	NaN |	NaN |	NaN |	NaN |	NaN |
| 2020-01-02 |	NaN |	NaN |	NaN |	NaN |	NaN |	NaN |	NaN |	NaN |
| 2020-01-03 |	NaN |	NaN |	NaN |	NaN |	NaN |	NaN |	NaN |	NaN |
| 2020-01-04 |	NaN |	NaN |	NaN |	NaN |	NaN |	NaN |	NaN |	NaN |
| 2020-01-05 |	NaN |	NaN |	NaN |	NaN |	NaN |	NaN |	NaN |	NaN |



#### Problem description
According to the docs (https://pandas.pydata.org/pandas-docs/stable/getting_started/dsintro.html#data-alignment-and-arithmetic):

> When doing an operation between DataFrame and Series, the default behavior is to align the Series index on the DataFrame columns, thus broadcasting row-wise

> In the special case of working with time series data, if the DataFrame index contains dates, the broadcasting will be column-wise

It seems to me that in both cases now the broadcasting is row-wise.

Is this an expected change for pandas 1.0.0 (I hope not - I never saw any FutureWarnings about it)? If so, the docs (and the examples) must be updated.

The same happens for the operators `-`, `/`, `*`, `%` 

#### Expected Output
Not sure if this is the expected output anymore, but it used to be equivalent to:

```python
df.add(series, axis=0)
```

| | a | b | c |
|---|-----|---|---|
| 2020-01-01 |	11 |	12 |	13 |
| 2020-01-02 |	21 |	22 |	23 |
| 2020-01-03 |	31 |	32 |	33 |
| 2020-01-04 |	41 |	42 |	43 |
| 2020-01-05 |	51 |	52 |	53 |

Although I can't replicate it, I'm pretty sure this was the behaviour until pandas 0.25.3

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.0.0
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.1.0.post20200127
Cython           : 0.29.14
pytest           : 5.3.4
hypothesis       : 4.54.2
sphinx           : 2.3.1
blosc            : None
feather          : None
xlsxwriter       : 1.2.7
lxml.etree       : 4.4.2
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.11.1
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : 1.3.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.2
matplotlib       : 3.1.1
numexpr          : 2.7.0
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.4
pyxlsb           : None
s3fs             : 0.4.0
scipy            : 1.3.2
sqlalchemy       : 1.3.13
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.7
numba            : 0.47.0

</details>
