is there any “objc standar/specification document” that you reference when implementing the objc parser?
No, Objective-C is implementation-defined.
There is documentation of parts of Objective-C on the clang site though, for example:
No Objective-C specific but here is the same for blocks:
BTW, perhaps it's just me but isn't it a bit weird that those Objective-C features are listed under languages extensions. Extensions of what? GCC? Since it's Clang (now days) that defines what the language is.