DOC: Document column order in MultiIndex.to_frame
```
conda create -q -n pandas1 python=3.6 pandas=0.23.4
source activate pandas1
```

```python
>>> import pandas as pd
>>> 
>>> pd.MultiIndex.from_tuples(
...     [("B1", "A1"), ("B2", "A1")], names=["B", "A"]
... ).to_frame()
        B   A
B  A         
B1 A1  B1  A1
B2 A1  B2  A1
```

```
conda create -q -n pandas2 python=3.5 pandas=0.23.4
source activate pandas2
```

```python
import pandas as pd

pd.MultiIndex.from_tuples(
    [("B1", "A1"), ("B2", "A1")], names=["B", "A"]
).to_frame()

        A   B
B  A         
B1 A1  A1  B1
B2 A1  A1  B2
```


#### Problem description
The ordering of columns from `pandas.MultiIndex.to_frame` varies in `python=3.5` vs `python=3.6` and the expected behaviour is not defined in the [docs](https://pandas.pydata.org/pandas-docs/stable/generated/pandas.MultiIndex.to_frame.html).

#### Expected Output

I would expect columns to be in the order of the `MultiIndex` levels. Barring that I would expect the columns to be lexicographical sorted, although I find that to be less intuitive.

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

<details>

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.6.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-33-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_CA.UTF-8
LOCALE: en_CA.UTF-8

pandas: 0.23.4
pytest: None
pip: 10.0.1
setuptools: 40.2.0
Cython: None
numpy: 1.15.1
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
```
```
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.6.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-33-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_CA.UTF-8
LOCALE: en_CA.UTF-8

pandas: 0.23.4
pytest: None
pip: 10.0.1
setuptools: 40.2.0
Cython: None
numpy: 1.15.1
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
```

</details>
