FindOpenGL: Update documentation

- Added intro code block showing how to use this module.
- Added examples section.
- Moved CMake versions that introduced specific target or variable
  to the item instead of above the list, as there are multiple versions
  and to have option to extend the items further in the future.
- Synced the module descriptions a bit with the other find modules.
- OpenGL_FOUND variable used (CMake 3.3). Uppercased OPENGL_FOUND is
  also set to the same value by the find_package_handle_standard_args().