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I've rewritten the "Commenting Code" section of the coding conventions as they were rather misleading, outdated and vague.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.
Should `@method` be supported to document magic methods? `@property-read` and `@property-write` can be considered, but don't know how widely they are supported.