Clarify docs re: ActiveRecord and verifying doubles

I found this confusing when I was getting started with verifying doubles. I've tried to clarify the language.

Thoughts/comments?
Implementing `respond_to_missing?` is important but doesn't solve the problem for instance doubles because RSpec doesn't have an instance to call `respond_to?` on.  For instance doubles it uses `method_defined?` which doesn't take into account `respond_to_missing?` (how could it?).