>API documentation is certainly copyrightable, even text definition file. A lot of creative process goes to putting braces, spaces, commas, naming

Yes but it is useless. You can e.g. copyright an electronics schematic. People aren't allowed to copy it and sell it but they can build the device that is being described by the schematic and then they can create their own schematic based on the device they have created. That's why we have patents.