docs: bodySizeLimit measures raw HTTP body (#94137)
Started from https://github.com/vercel/next.js/discussions/93989
Noticed we even have a test helper to account for overhead:
https://github.com/vercel/next.js/blob/fa4c0def823447e75366153690abeda79c7c4ec2/test/e2e/app-dir/actions/account-for-overhead.js#L1-L11
Let's document that the flag controls the raw http body size, overhead +
data
