docs: update trademark disclaimer
It's a long time since SGI was the copyright holder for the OpenGL trademark. And we implement more APIs by now, so let's update the disclaimer to instead redirect to the Khronos licensing page for details.
While we're at it, soften the language on legal status as a formal implementation, as we currently have conformant drivers for most of the APIs by now. But refer to the Khronos website for details, as conformance status for drivers are subject to change.