Improve discoverability of bubbling

[UkoeHB]

How can Bevy's documentation be improved?

The Observer and picking docs should mention bubbling and point to where you can learn more about it. The Pointer type just silently bubbles without any indication in the docs unless you know where to look (it's the Event implementation).

If you type bubble into the doc search, nothing shows up. I had to hunt down a bubbling-related PR to find a relevant keyword (Traversal), and then a lucky doc link led me to Trigger::propagate, where a very confusing doc link (Event::Traversal) then led me to the Event docs.