docs(workers): warn that background async needs await or waitUntil

[HaoChiBao]

Summary

• Add a short caution on ctx.waitUntil() explaining that async work which is neither awaited nor passed to waitUntil() may be canceled when the invocation ends.

Fixes #32086.

Why

Developers often start background tasks and return a response assuming the task continues. Without await or ctx.waitUntil(), the runtime can interrupt that work — dropping logs or leaving writes incomplete. The page already documents how to use waitUntil; it did not warn about the failure mode when it is omitted.

Test plan

• Preview /workers/runtime-apis/context/#waituntil: caution renders above the common-uses list
• Existing 30-second limit and example code unchanged

[cloudflare-docs-bot]

Review

💡 1 suggestion found in commit 261f929.

👉 Fix in your agent 👈

Fix the following review findings in PR #32626 (https://github.com/cloudflare/cloudflare-docs/pull/32626).

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---

## Style Guide Review

### Suggestions (1)

#### SG-d90e38b12068 · Serial comma
- **File:** `src/content/docs/workers/runtime-apis/context.mdx` line 212
- **Issue:** A list of three participial phrases (`dropping logs, leaving writes unfinished, or failing silently`) is joined by `or` without a comma before the final conjunction.
- **Fix:** Add a serial comma before `or`: `dropping logs, leaving writes unfinished, or failing silently` → `dropping logs, leaving writes unfinished, or failing silently`.

Code Review

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_{No code review issues found.}

Conventions

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Style Guide Review

Suggestions (1)

File	Issue
workers/runtime-apis/context.mdx line 212	Serial comma — A list of three participial phrases (dropping logs, leaving writes unfinished, or failing silently) is joined by or without a comma before the final conjunction. Fix: Add a serial comma before or: dropping logs, leaving writes unfinished, or failing silently → dropping logs, leaving writes unfinished, or failing silently.

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