FindGLUT: Update documentation

- Synced module documentation with other similar find modules.
- Added examples section.
- Refactored the obsolete variables section to only mention in the
  result variables that GLUT_INCLUDE_DIRS should be used instead of
  the previously misused cache variable GLUT_INCLUDE_DIR. The
  GLUT_INCLUDE_DIR as such is not actually deprecated/obsolete as it is
  still used to find GLUT (only its previous usage way is
  changed/obsolete).