2.0 docblock coding conventions

Refs https://github.com/cakephp/cakephp/issues/4559

I've rewritten the "Commenting Code" section of the coding conventions as they were rather misleading, outdated and vague.
Thanks @dereuromark and @thaJeztah for their inputs.
I still need to teach new colleagues that English also applies to, e.g. Naming your *variables*. They happily document code in English, but use Dutch for naming database-columns and variables, so I don't think it hurts (but I don't know if it has been a problem in the past with *CakePHP* contributors?)