docs: note that a pending snippet skips SSR of boundary content (#18922)

also remove an outdated note about needing a pending snippet for await

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Co-authored-by: Satvik Mishra <satvikmishra08@gmail.com>
Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

documentation/docs/05-special-elements/01-svelte-boundary.md

@@ -38,7 +38,7 @@ This snippet will be shown when the boundary is first created, and will remain v
 
 The `pending` snippet will _not_ be shown for subsequent async updates — for these, you can use [`$effect.pending()`]($effect#$effect.pending).
 
-> [!NOTE] In the [playground](/playground), your app is rendered inside a boundary with an empty pending snippet, so that you can use `await` without having to create one.
+> [!NOTE] During server-side rendering, a boundary with a `pending` snippet renders that snippet and ignores the rest of its content. See [Server-side rendering](await-expressions#Server-side-rendering).
 
 
 ### `failed`