Additional examples around element overloading

[LeaVerou]

This is a followup to #502.

• It might make sense to mention distinct elements sharing the same interface as a halfway solution, with the positive example of <video> and <audio> as instances of HTMLMediaElement.
• <h1> to <h6> are examples of insufficient overloading.
• @hober also mentioned that <legend>, <caption> and <figcaption> should have been the same element.
• <input type=checkbox switch> is a positive example of overloading (see New principle: Avoid introducing mode errors #532)