poor melt example in documentation
#### Problem description

The example chosen to illustrate `melt` 

https://pandas.pydata.org/pandas-docs/stable/user_guide/reshaping.html#reshaping-by-melt

is not ideal. Normally, melt would be used to convert from wide to long data. However, the data on the left hand side of the example is already in long format (each variable corresponds to a different column) and the melt command is just creating a strange "thing" where the column value contains two different variables. I'm using this figure in my teaching as an example of what NOT to do when reshaping data.

A better example would be  (taken from: https://www.jstatsoft.org/article/view/v059i10)

first | last | Treatment A | Treatment B
-----|------|------------------|---------------
Jane |Doe| 16 |11
Mary |Johnson| 3| 1

melted into:

first | last | variable | value
-----|------|------------------|---------------
Jane |Doe| Treatment A| 16 
Jane |Doe| Treatment B| 11
Mary |Johnson| Treatment A| 3 
Mary |Johnson| Treatment B| 1


I'm happy to provide a pull request against the docs if this is considered an improvement.