DOC: Revise merging.rst with graphical examples
Closes #9301. Though further table size adjustment / plotter module refactoring is required, I'd like to ask any feedbacks for current output.

I've prepared table plotter under utils.doctools, but is there anywhere more appropriate?

cc @JanSchulz 

![merge_update png](https://cloud.githubusercontent.com/assets/1696302/7507301/d0f7f6da-f4a8-11e4-98db-ecbdb86bd636.png)
