Improve documentation for attributes option

[edwardhorsford]

The attributes option on some macros is seemingly very powerful - but I'm not sure the expected format is documented anywhere.

The docs say:
"HTML attributes (for example data attributes) to add to the hint span tag."

As a new user, it would have been helpful to see something that showed the data structure needed - I had to guess it would be an object with keys.

[edwardhorsford]

I was hoping to use attributes to set a class on a checkbox item - but I don't think I can because there's already one. Should attributes be merged in? should it mention that there's some stuff you can't set?

[NickColley]

@edwardhorsford I think at this point it'd be easier to consider adding 'classes' to individual items than merging attributes (as this would require Nunjucks global functions), so if you need that feel free to raise a separate issue.