DOC: Clarify rank documentation
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Example of top/bottom behaviour vs ascending:
```python
x = np.array([np.nan, 3.1, 1.4, 2.1, np.nan, 5, 0.5])
pd.Series(x).rank(ascending=False, na_option='bottom', method='min') ->
0    6.0
1    2.0
2    4.0
3    3.0
4    6.0
5    1.0
6    5.0
dtype: float64

pd.Series(x).rank(ascending=True, na_option='bottom', method='min') ->
0    6.0
1    4.0
2    2.0
3    3.0
4    6.0
5    5.0
6    1.0
dtype: float64

pd.Series(x).rank(ascending=True, na_option='top', method='min') ->
0    1.0
1    6.0
2    4.0
3    5.0
4    1.0
5    7.0
6    3.0
dtype: float64

pd.Series(x).rank(ascending=False, na_option='top', method='min') ->
0    1.0
1    6.0
2    4.0
3    5.0
4    1.0
5    7.0
6    3.0
dtype: float64
```