[ci skip] Changes some wording to better foreshadow what is coming up later

This should prevent some anxiety readers may experience when the doc goes into something different than what it just explained. Also clarifies a statement about priority in engines and application environments
s/these/them here, no ? Could you please [squash your commits](http://guides.rubyonrails.org/contributing_to_ruby_on_rails.html#squashing-commits) together ? Also, could you tell me once you've updated your patch please ? :-) Pushing a commit won't create a notification ; then we forget about PRs. Thanks!