[whatwg] Clarify how to indicate document hierarchy

[Benjamin Hawkes-Lewis]

Sorry, Evolution cut me off. Example 3 was from:

http://www.w3.org/TR/html401/struct/tables.html#h-11.2.1

Of course, just because the HTML4 spec goes in for this stuff doesn't
itself mean it's a good idea.

--
Benjamin Hawkes-Lewis