Update documentation of Diagrams example

[ivanperez-keera]

The diagrams example https://github.com/ivanperez-keera/Yampa/blob/8f703cd4fe1af2e4dcaed02cd2743760343e90f9/yampa/examples/Diagrams.hs has outdated documentation (it refers to cabal v1 commands without the prefix), and the definitions are not documented.

Updating the documentation would help especially newcomers use that example.