Air Canada (and Westjet, but that's not relevant here) should add a bold-faced, plain language overbooking notice on the purchase page of the website right alongside the cancellation rules, with a possible link to a longer plain language explanation elsewhere on the website.

Currently there is nothing. While I do expect people to be responsible for informing themselves, it isn't reasonable to expect people to the read hundreds of pages of the general tariff. And the only other mention of it is on the FAQ, but someone who is ignorant of overbooking as a concept wouldn't even conceive of it as a question to be asked.