Document the named subject feature.

Closes #1566
`it defines a memoized helper method` suggests this:

``` ruby
subject { your_logic }
def name; @name ||= subject; end
```

...but actually it's:

``` ruby
subject { your_logic }
alias name subject
```

There are differences to these; for example, if someone defines a `subject` method after this, the former `name` will get the value of the new `subject`, while the latter is an alias and will not be changed a `subject` redefinition.

I think it's worth explicitly saying it's an alias here instead of the phrasing you currently have.