Small JavaDoc corrections.

http://libgdx.badlogicgames.com/nightlies/docs/api/com/badlogic/gdx/Input.html#getRotationMatrix-float:A- links to http://developer.android.com/reference/android/hardware/SensorManager.html#getRotationMatrix%28float[],%20float[],%20float[],%20float[]%29 which describes the axes a bit different than our JavaDoc.