Document that the default for `rails server -b` has changed

I want to cut down the number of blockers before beta2, so here is a quick fix for #16578.

We can just as easily keep the old default on Rails side, but after a quick chat with @jeremy we are leaning towards using the new default from Rack.

cc @rafaelfranca @dhh @jeremy @tenderlove @matthewd @NZKoz