Add documentation for boolean attributes

[patrick-steele-idem]

Input data:

{
    options: [
        { 
            value: 'red',
            selected: false
        },
        { 
            value: 'green',
            selected: true
        },
        { 
            value: 'blue',
            selected: false
        }
    ]
}

Input Marko template:

<select>
    <option value="${option.value}" selected="${option.selected}" for="option in data.options">
        ${option.value}
    </option>
</select>

Output HTML:

<select>
    <option value="red">red</option>
    <option value="green" selected>green</option>
    <option value="blue">blue</option>
</select>