chore(metadata): add clarifying comment (#69549)
## Why?
Adding a clarification comment in case somebody finds this the
[warning](https://github.com/vercel/next.js/blob/397a3eb475c75a38ab6178e269e0bbb3d1dd84f7/packages/next/src/lib/metadata/resolvers/resolve-opengraph.ts#L137)
in their Vercel Deployment, and goes back to the code to see why they're
seeing it.
