Document the named subject feature.

Closes #1566
I was trying to combine two sentences into one. Specifically this from the [`let` and `let!` docs](https://www.relishapp.com/rspec/rspec-core/v/3-0/docs/helper-methods/let-and-let):

> Use `let` to define a memoized helper method. The value will be cached across
multiple calls in the same example but not across examples.