DOC/API: clarify usage of where, mask broadcast arguments
#### Code Sample, a copy-pastable example if possible

```python
>>> x = pd.Series(0, pd.MultiIndex.from_product([[0], [1, 2]]))
>>> y = pd.Series(True, [0])
>>> x.where(y, level=0)

Traceback (most recent call last):
  File "bug.py", line 1, in <module>
    x.where(y, level=0)
  File "~/anaconda3/lib/python3.5/site-packages/pandas/core/generic.py", line 4813, in where
    raise_on_error)
  File "~/anaconda3/lib/python3.5/site-packages/pandas/core/generic.py", line 4557, in _where
    cond, _ = cond.align(self, join='right', broadcast_axis=1)
  File "~/anaconda3/lib/python3.5/site-packages/pandas/core/series.py", line 2350, in align
    broadcast_axis=broadcast_axis)
  File "~/anaconda3/lib/python3.5/site-packages/pandas/core/generic.py", line 4419, in align
    fill_axis=fill_axis)
  File "~/anaconda3/lib/python3.5/site-packages/pandas/core/generic.py", line 4488, in _align_series
    return_indexers=True)
  File "~/anaconda3/lib/python3.5/site-packages/pandas/indexes/base.py", line 2646, in join
    return_indexers=return_indexers)
  File "~/anaconda3/lib/python3.5/site-packages/pandas/indexes/base.py", line 2738, in _join_multi
    raise ValueError("cannot join with no level specified and no "
ValueError: cannot join with no level specified and no overlapping names

>>> x = pd.DataFrame(0, pd.MultiIndex.from_product([[0], [1, 2]]), [3])
>>> y = pd.Series(True, [0])
>>> x.where(y, axis=0, level=0)

    3
0 1 NaN
  2 NaN

>>> y = pd.Series(True, [3])
>>> x.where(y, axis=1)

      3
0 1 NaN
  2 NaN
  
>>> y = pd.DataFrame(True, [0], [3])
>>> x.where(y, level=0)

Traceback (most recent call last):
  File "bug.py", line 3, in <module>
    x.where(y, level=0)
  File "~/anaconda3/lib/python3.5/site-packages/pandas/core/generic.py", line 4813, in where
    raise_on_error)
  File "~/anaconda3/lib/python3.5/site-packages/pandas/core/generic.py", line 4557, in _where
    cond, _ = cond.align(self, join='right', broadcast_axis=1)
  File "~/anaconda3/lib/python3.5/site-packages/pandas/core/frame.py", line 2816, in align
    broadcast_axis=broadcast_axis)
  File "~/anaconda3/lib/python3.5/site-packages/pandas/core/generic.py", line 4414, in align
    fill_axis=fill_axis)
  File "~/anaconda3/lib/python3.5/site-packages/pandas/core/generic.py", line 4436, in _align_frame
    other.index, how=join, level=level, return_indexers=True)
  File "~/anaconda3/lib/python3.5/site-packages/pandas/indexes/base.py", line 2646, in join
    return_indexers=return_indexers)
  File "~/anaconda3/lib/python3.5/site-packages/pandas/indexes/base.py", line 2738, in _join_multi
    raise ValueError("cannot join with no level specified and no "
ValueError: cannot join with no level specified and no overlapping names

```
#### Problem description

`where()` and `mask()` ignore the broadcasting arguments `axis` and `level`

#### Expected output

```python
0  1    0
   2    0
dtype: int64

    3
0 1 0
  2 0

    3
0 1 0
  2 0

    3
0 1 0
  2 0
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.8-100.fc24.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: C
LANG: C
LOCALE: None.None

pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.11.3
scipy: 0.18.1
statsmodels: 0.8.0
xarray: 0.9.1
IPython: 4.2.0
sphinx: 1.5.1
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: 3.3.0
numexpr: 2.6.2
matplotlib: 2.0.0
openpyxl: 2.4.1
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.2
bs4: 4.5.3
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: 1.1.5
pymysql: None
psycopg2: None
jinja2: 2.9.4
boto: 2.45.0
pandas_datareader: None
</details>
