text: Title: [DOCS] preprocessing API gone
  Body: Probably when we preprocessing part of the library got split up into a folder we forgot to make sphinx aware of it. Docs look like this now; 
  
  ![image](https://user-images.githubusercontent.com/1019791/74315460-c3d22e80-4d77-11ea-8a84-23d159443edc.png)
  