DOC: pandas.DataFrame.mask
#### Location of the documentation

https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.mask.html#pandas-dataframe-mask

#### Documentation problem

The first line of docstring states:

>> Replace values where the condition is True.

However, `where` replaces the values for which the `cond` is False.

e.g.
```
zeros = pd.DataFrame(np.zeros(5))
zeros.where(zeros == 0.0, "REPLACED")
>>>
0.0
0.0
0.0
0.0
0.0


zeros.where(zeros != 0.0, "REPLACED")
>>>
REPLACED
REPLACED
REPLACED
REPLACED
REPLACED



#### Suggested fix for documentation

I think documentation should say "Replace values where the condition is False."

The rest of the `where` documentation should also be changed to reflect the actual behaviour of `where`.
