Auto merge of #79742 - GuillaumeGomez:move-tooltips-messages-out-of-html, r=Nemo157

Auto merge of #79742 - GuillaumeGomez:move-tooltips-messages-out-of-html, r=Nemo157

Move tooltips messages out of html

First thing first: nothing in the output has changed. You still have the "i" on the left of code blocks examples when they have `ignore`, `compile_fail`, `should_panic` and `edition`. The behavior also remains the same: when you hover the "i", you have the corresponding message showing up.

So now, why this PR then? I realized recently that we were actually generating those messages into the HTML every time whereas all messages are the same (except for the edition ones, I'll come back to it later). So instead of generating more content, I simply moved it inside the CSS thanks to pseudo elements (`::before` and `::after`). The message is now inside `::after` and we use the `::before` to have the small triangle on the left of the message. So now, we have less HTML generated which is seems pretty nice.

So now, back to the `edition` change: the message is globally the same, but the "edition" itself can be different (2015 or 2018 currently, I expect 2021 to arrive not too far in the future). So the only difference for it is that I added a new attribute on the tooltip called `edition` which contains this information. Then, the `::after` uses it inside its `content` (you can get the content of an element's attribute by using `attr` and concat different strings by simply having them after the other).

Don't hesitate if a part of my explanations isn't clear.

r? `@jyn514`