docs(sandboxes): note that a JavaScript sandbox needs an import to apply

Cursor Bugbot pointed out that `useRemoteSandbox` keys on detected static
import/require and never on the selected sandbox, so JavaScript without one runs
locally and the selection has no effect.

Keeping the behaviour: honouring the selection would force those blocks remote,
and the large-value-ref guard immediately below would then reject code that runs
fine today. Documenting it instead, next to the picker, since a selection that
silently does nothing is only surprising if nothing says so.

Python is unaffected — it always runs remotely, so its sandbox always applies.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: icecrasher321

apps/docs/content/docs/en/workflows/blocks/function.mdx

@@ -100,6 +100,11 @@ pandas
 
 Then open the block's advanced options and choose the sandbox under **Sandbox**.
 
+In JavaScript the sandbox applies to code that uses `import` or `require` — that is
+what sends the block to a remote sandbox in the first place, so a block without them
+keeps running locally and ignores the selection. Python always runs remotely, so a
+selected sandbox always applies.
+
 <Callout type="info">
 Two sandboxes with the same language and the same package list share one build,
 so duplicating a set costs nothing. Editing a package list starts a new build;