Document the named subject feature.

Closes #1566
Gotcha.  I'd feel better about that doc being in our API docs:

http://rubydoc.info/github/rspec/rspec-core/RSpec/Core/MemoizedHelpers/ClassMethods#subject-instance_method

Adding a note there doesn't add the additional maintenance burden that this extra scenario adds.  It also feels more appropriate to document something that we don't support via a note in the docs than in an executable example that adds to the travis build times.

Thoughts?