Document Promise handling in JavaScript/TypeScript code execution

Clarifies that JS/TS code returning Promises is automatically awaited,
ensuring users understand they receive resolved values in results.

Related to cloudflare/sandbox-sdk#246

Author: github-actions[bot]

src/content/docs/sandbox/api/interpreter.mdx

@@ -90,6 +90,25 @@ console.log(result.results[0].text); // "15"
 </TypeScriptExample>
 :::
 
+**Promise handling in JavaScript/TypeScript**:
+
+When executing JavaScript or TypeScript code, if the last expression evaluates to a Promise, it is automatically awaited. This allows you to write async code and receive resolved values in results.
+
+<TypeScriptExample>
+```
+const ctx = await sandbox.createCodeContext({ language: 'javascript' });
+
+const result = await sandbox.runCode(`
+async function fetchData() {
+  return { status: 'success', value: 42 };
+}
+fetchData()
+`, { context: ctx });
+
+console.log(result.results[0].json); // { status: 'success', value: 42 }
+```
+</TypeScriptExample>
+
 **Error handling**:
 
 <TypeScriptExample>