docs: move some $state information around

[Rich-Harris]

Was just reading https://svelte.dev/docs/svelte/$state and I really don't like the 'Class instances are not proxied' breakout quote. It doesn't make sense to disrupt the flow with noise about classes at this point, and it's redundant — the very next sentence is "State is proxified recursively until Svelte finds something other than an array or simple object".

Verdict: get rid of it

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