docs(guides): add two v4 breaking changes

[tlhunter]

Adding two notes from an application update of mine where I needed to make the following changes:

-      reply.res.setHeader('Content-Type', 'image/png');
-      qrcode.toFileStream(reply.res, dest.url);
+      reply.raw.setHeader('Content-Type', 'image/png');
+      qrcode.toFileStream(reply.raw, dest.url);
+
+      return reply;

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm

[github-actions]

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