Updated loop documentation with explicit example of hash usage

For #2929; changes example from:

    {{ item.subKeyName }}

To:

    - name: add several users
      action: user name={{ item.name }} state=present groups={{ item.groups }}
      with_items:
        - { name: 'testuser1', groups: 'wheel' }
        - { name: 'testuser2', groups: 'root' }