add eval examples
xref #31952 

Screenshot for `pandas.eval`:
![image](https://user-images.githubusercontent.com/33491632/74459941-04b76980-4e84-11ea-9f1b-2b6ad9cb5494.png)

Screenshot for `pandas.DataFrame.eval`:
![image](https://user-images.githubusercontent.com/33491632/74460007-23b5fb80-4e84-11ea-931b-0f73eebb6725.png)

Great thanks @MarcoGorelli 