`not()` should be explained in Python doc

`not()` function works like [not](https://docs.python.org/3/reference/lexical_analysis.html#keywords) keyword as shown below:

```python
print(not())      # True
print(not(False)) # True
print(not(True))  # False

print(not True)  # False
print(not False) # True
```

But Python doc doesn't explain `not()` function so `not()` should be explained in Python doc.