DOC: Indexing and selecting data and the use of mask/where
#### Location of the documentation

https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#evaluation-order-matters

#### Documentation problem

The documentation talks about how **not** to use `dfb['c'][dfb['a'].str.startswith('o')] = 42` for assignment of multiple values to a `pandas.Series` or `pandas.DataFrame` column. it does however fail to make a concrete suggestion how to do it given the example. 

#### Suggested fix for documentation

I suggest to mention the `pandas.Series.where` and `pandas.Series.mask` methods here in order to show how the assignment can be done, i.e. by adding an example like:

```python
dfb['c'].mask(dfb['a'].str.startswith('o'), 42)
```
which performs the desired assigment of rows matching `o` to `42`. I am happy to contribute this as a PR myself.