Passing DataFrame of values to fillna()
- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the latest version of pandas.

I'm not sure if this is a bug, a documentation issue, or just user error.
I found related issues #9750 #29048 that help me get the desired output but still don't explain why behavior doesn't match expectation based on my reading of the docs.

#### Code Sample

Following #9750 here's some code to reproduce the issue. 

```python
df = pd.DataFrame([[1, 1, 1],[2, 1, 1],[2, 1, 1],
                   [np.nan, np.nan, np.nan]], columns=["a","b","c"])
df.fillna(df.mode())
```
The resulting DataFrame is just `df` itself, i.e.  without any null values filled. 

#### Problem description

I understand from the issues I mentioned (and numerous StackOverflow answers) that this is due to `df.mode()` returning a `DataFrame`, as opposed to a `Series`, (which is why e.g. `df.mean()` gives the expected results). To get the desired output `df.fillna(df.mode().iloc[0])` works in this case. 

But I'm confused by [the docs](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.fillna.html) which describe the value parameter:

> value: scalar, dict, Series, or DataFrame

    Value to use to fill holes (e.g. 0), alternately a dict/Series/DataFrame of values 
    specifying which value to use for each index (for a Series) or column (for a 
    DataFrame). Values not in the dict/Series/DataFrame will not be filled. This value 
    cannot be a list.

From how I read this, you should be able to pass a DataFrame with the same column names and have values filled for those columns. 

Just in case the passed in `DataFrame` needed to be the same shape, I tried

```python
df2 = pd.DataFrame(np.zeros((4, 3)))
df.fillna(df2)
```
 But still got `df` back.

I read the source but the answer still eludes me.


#### Expected Output

The expected behavior is that the missing values will be filled with mode of each column.

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

<details>

INSTALLED VERSIONS
------------------
commit           : b5958ee1999e9aead1938c0bba2b674378807b3d
python           : 3.8.5.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.6.0
Version          : Darwin Kernel Version 19.6.0: Mon Aug 31 22:12:52 PDT 2020; root:xnu-6153.141.2~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.5
numpy            : 1.19.2
pytz             : 2020.5
dateutil         : 2.8.1
pip              : 20.3.3
setuptools       : 51.0.0.post20201207
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None
</details>
