i really dont think its necessary to have too much explaining on the matter. i doubt it will be a major plot point. i think it may be limited to something along the lines where yoda, obi-wan and mace are the only jedi left and mace says something like "then it is over. the jedi will cease to exist." and yoda says, "no, there is another way." probably a little more than that, but i doubt it will be a major plot point in Episode III. just enough so we understand what obi-wan is doing when he vanishes as vader goes for his head