docs: clarify svelte:component migration, avoids common gotcha

[rgon]

Clarify docs to specify that, while svelte:component is no longer needed and deprecated, capitalizing the component variable name is required to avoid mismatching types.

Avoids users doing the same gotcha that I stumbled upon, it will be a common oversight when refactoring existing codebases: #13795

As evidenced in these docs themselves (corrected):

...
	let component: Component<{ foo: string }> = $state(
		Math.random() ? ComponentA : ComponentB
	);
</script>

<svelte:component this={component} foo="bar" />