Document the named subject feature.

Closes #1566
The feature is demonstrating that subject is only executed once, if you want to cover this without covering memoization consider: 

```Ruby
let(:array) { [1,2,3] }

subject { array.pop }

it "executes subject once per example" do
  2.times { subject }
  expect(array).to eq [1,2]
end
```