Suffixes not applied to join columns in pd.merge
```python
import pandas as pd

left = pd.DataFrame({"a": [1.0, 2.0]})
right = pd.DataFrame({"a": [1.0]})

pd.merge(left, right, how="left", on="a", suffixes=("_left", "_right"))
#      a
# 0  1.0
# 1  2.0
```

#### Problem description

The documentation for `suffixes` in `pd.merge` reads:
```
suffixes : tuple of (str, str), default (‘_x’, ‘_y’)
Suffix to apply to overlapping column names in the left and right side, respectively. To raise an exception on overlapping columns use (False, False).
```
but it seems that the join columns themselves are ignored.  I'm guessing this is by design, but if so should the docstring be updated to reflect this?

#### Expected Output

```python
#    a_left  a_right
# 0     1.0      1.0
# 1     2.0      NaN
```