DOC: intended series.loc behavior undocumented for boolean series
#### series.loc when passed a boolean series
How will series.loc behave when passed a boolean series whose index contains the same values, but in a different order, than the focal series' index?

```python
s1 = pd.Series(list('ab'))
mask = [True, False]
s2 = pd.Series(mask, pd.Index([1,0]))

print(s1.loc[mask])
print(s1.loc[s2])
```
0    a
dtype: object
1    b
dtype: object

#### Problem description

I expected the index of `s2` to be ignored, in effect treating `s2` as a boolean array input, but I was wrong.

The [documentation on master](https://pandas-docs.github.io/pandas-docs-travis/) doesn't list a (boolean) series as an allowed input, hence doesn't provide guidance on which behavior should be expected/is intended. 

What is the intended behavior? If given guidance, I'll update the documentation.

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.14.152-127.182.amzn2.x86_64
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.3
numpy            : 1.17.3
pytz             : 2019.3
dateutil         : 2.7.4
pip              : 19.3.1
setuptools       : 40.4.3
Cython           : 0.29.14
pytest           : 5.3.2
hypothesis       : None
sphinx           : 2.3.0
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10
IPython          : 7.0.1
pandas_datareader: None
bs4              : 4.8.1
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.1
numexpr          : 2.7.0
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.4.0.dev0+4dcf606
sqlalchemy       : 1.3.11
tables           : None
xarray           : 0.14.1
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None

</details>

