docs: mention that snippets can only be called through @render

[HoldYourWaffle]

See #15517.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15518-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15518

[Rich-Harris]

Thanks for the PR. With the warning in #15521 I think the documentation change is unnecessary — people will find out when they need to find out — so I'll close this

[HoldYourWaffle]

Understandable, thanks for all the great work!

[axel7083]

Thanks for the PR. With the warning in #15521 I think the documentation change is unnecessary — people will find out when they need to find out — so I'll close this

Hey @Rich-Harris, this piece of documentation would have been helpful because sometime we don't test with snippet our component, and people will provide it as props (just happened in our codebase), and this scenario could have been catch if the documentation was mentioning this