DOC: Example for plotting scatterplot matrices is incomplete
#### Location of the documentation

https://pandas.pydata.org/pandas-docs/stable/user_guide/visualization.html#scatter-matrix-plot

#### Documentation problem

In the above-mentioned documentation, the following code is suggested to build/display a scatterplot matrix:
```
df = pd.DataFrame(np.random.randn(1000, 4), columns=["a", "b", "c", "d"])
scatter_matrix(df, alpha=0.2, figsize=(6, 6), diagonal="kde");
```

This code indeed builds something, and returns a numpy array, but it does not display the graphics.

If one does not have a look at the source code or at one a the very few other docs on the subject, like:
https://stackoverflow.com/questions/7941207/is-there-a-function-to-make-scatterplot-matrices-in-matplotlib
https://stackoverflow.com/questions/34347145/pandas-plot-doesnt-show
Or if one is a newbie in Python/DataScience/Pandas and does not know about matplotlib. One will never know how to get the graphics to display. And neither the numpy array returned nor the docs/proposed-completions proposed by most IDE will give you any hint of the problem and/or its solution :{ 

#### Suggested fix for documentation

- Suggested correction to the doc: add the fact that to display the graph you need both 1/ to import an external module & 2/ to call a method of this module
 
```
import matplotlib.pyplot as plt, plt.show()
```

- Why? Because documentation examples should be self sufficient and provide MVE.

PS: I've not looked at the other type of graphs' example, but the problem might be the same for their MVE.