docs: added additional explanation for valid mutation and re-assignment of $state runes.

[yail259]

As title

[changeset-bot]

⚠️ No Changeset found

Latest commit: bd08827

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-15070-svelte.vercel.app/

this is an automated message

[Rich-Harris]

thank you but there's already a section on this: https://svelte.dev/docs/svelte/$state#Passing-state-into-functions