docs(routing): update error handling description

[azrikahar]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

If the route starts with /api/ (either because it is placed in api/ or routes/api/) the default will change to application/json and a JSON object will be sent.

I believe the above excerpt is no longer applicable due to the changes from #3002.

It was true when the response still uses a util function called isJsonRequest and it has a specific check for event.path.startsWith("/api/") to make sure routes in that path will return JSON:

nitro/src/runtime/internal/error/utils.ts

Lines 11 to 24 in e8d6099

	export function isJsonRequest(event: H3Event) {
	// If the client specifically requests HTML, then avoid classifying as JSON.
	if (hasReqHeader(event, "accept", "text/html")) {
	return false;
	}
	return (
	hasReqHeader(event, "accept", "application/json") ||
	hasReqHeader(event, "user-agent", "curl/") ||
	hasReqHeader(event, "user-agent", "httpie/") ||
	hasReqHeader(event, "sec-fetch-mode", "cors") ||
	event.path.startsWith("/api/") ||
	event.path.endsWith(".json")
	);
	}

but as it was removed and #3002 specifically split the error handler into dev and prod handlers, I've opted to update the docs based on the new error handling changes.

That being said, should the following existing sentence:

This behaviour can be overridden by some request properties (e.g.: Accept or User-Agent headers).

be removed since it might be redundant now? or at least the User-Agent part might not really work, assuming if I didn't misunderstand the new error handling logic of course.

Update: in hindsight maybe this PR should've targeted v2 instead of v3 since #3002 was merged into v2. Do let me know if you'd prefer that.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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