I think so.  Having easily findable docs for what `subject` does when called from an example seems important.

That said, given these ordering dependencies, it makes me think we should make the module here mange it (e.g. via an `included` hook), rather than forcing ExampleGroup to include/extend things in a particular order.  Also, given the similarity between `subject` and `let`, and the fact that one now depends on the other...I wonder if we should combine these into one module in one file?  That'd have a nice side benefit of loading one fewer file at runtime when rspec is loading, leading to ever-so-slightly-faster (but barely noticeable if at all) runtimes.