Padrino core documentation cleanup

These pull request extracted only the document changes from [my previous pull request#1389](https://github.com/padrino/padrino-framework/pull/1389)

I wasn't able to extract the right commits of my fork and I somehow messed it up. Please take only my commits downwards from SHA cf50b93.

These are the commits of the documentation clean up of [pull request](https://github.com/padrino/padrino-framework/pull/1389)