BUG: boolean indexing error with .drop()
#### Code Sample, a copy-pastable example if possible
```python
df = pd.DataFrame( data = {
                         'acol'  : np.arange(4),
                         'bcol' :  2*np.arange(4)
                        })
df.drop(df.bcol > 2, axis=0, inplace=True)

print(df)
```

#### Expected Output
```python
	acol	bcol
0	0	0
1	1	2
```

#### Observed Output 
```python
	acol	bcol
2	2	4
3	3	6
4	4	8

```

#### Problem description
The anticipated behavior was that rows with `bcol` > 2 would be dropped.  The actual behavior is that the boolean gets converted to 0/1, and then treated as index label.  So *row numbers* 0 and/or 1 are dropped... but all other rows will be kept.

The documentation did not make it clear what was happening.

Solutions might include documentation clarifying that `.drop()` cannot be used with boolean indexing, or a warning when receiving the (attempted) boolean index.


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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Linux
OS-release: 2.6.32-573.12.1.el6.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.2
pytest: 3.1.2
pip: 9.0.1
setuptools: 33.1.1.post20170320
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.1
xarray: 0.9.6
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.0
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.5.0a1
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.8.0
bs4: 4.5.3
html5lib: 0.9999999
sqlalchemy: 1.1.11
pymysql: 0.7.9.None
psycopg2: 2.7.1 (dt dec pq3 ext lo64)
jinja2: 2.9.5
s3fs: 0.1.1
pandas_gbq: None
pandas_datareader: None
</details>
