show-hide-content should work with new radio/checkbox markup

[robinwhittleton]

• update the selectors in show-hide-content to work with both the new and original markup. Simply removing the .block-label left it feeling a bit unscoped, so I’ve added a direct child selector. This potentially could cause problems, but it works with our recommended markup old and new at least.
• update the spec to follow the new markup.
• update the documentation to recommend the new markup.

Things to consider:

• are the new selectors too restrictive?
• should we be testing both old and new markup?
• how could we decouple this in future further?

[gemmaleigh]

I'm keen to merge this as the examples of 'Conditionally revealing content' are currently broken for GOV.UK elements.

Let's get this in and bump the toolkit, if there are suggestions for a better way of achieving this, we can roll those out later.