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I've rewritten the "Commenting Code" section of the coding conventions as they were rather misleading, outdated and vague.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.
@thaJeztah for such questions I normally consult the existing codebase and look for issues/pull requests in the past.