Add more details to spread props note section

[toocomputer]

The Note section on the spread props page mentions $$props, but that concept isn't discussed prior to that point, so it would be helpful to users to describe what it is before mentioning its use case.

The note says "...you can do so by accessing $$props directly", but if the user doesn't even know what $$props is how would they know how to access it?

The user shouldn't have to look at an outside resource to learn that "$$props is an internal variable that holds all the props received by a Svelte component".