docs: clarify on:event usage

[cloudymeatball]

Problem 1: The events emitted/forwarded were not listened for.
Fix 1: Changed event names to show consistency amongst examples

Problem 2: Why events need to be forwarded is unclear (on this page). createEventDispatcher docs does mention custom events dont bubble. But components also cannot listen to DOM events that do bubble.
Fix 2: Clarified that components only listen to event from their direct children

Please close #10087 as I tried to merge svelte-4 into main (my bad)

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[dummdidumm]

This is not completly accurate, if you have something like

<div on:click={foo}
  <Button />
</div>

and you click on the button then the foo handler would also get notified. I'm not sure right now how to bring this nuance across.

[cloudymeatball]

I think in this particular case, it works because DOM events bubble through DOM tree and "bypasses" the <Button> component. So imo this would be an element directive not component directive.

But I noticed this doesn't trigger. https://svelte.dev/repl/404bf36cde154bceb7ac7b0852e89d66?version=4.2.8

<Wrapper on:input={foo}>
  <Input on:input />
</Wrapper>

while if placeholder slot is <Input on:input /> then it works: https://svelte.dev/repl/31fdd56cc6754245a463033f910f14bb?version=4.2.8

Also just double checking when we use the word parent, do we refer to the component/page in which a component is instantiated, or do we simply mean in terms of hierarchy. Like in my REPL, is the parent of <Input> the <App> or <Wrapper>?