chore: remove asset prefix warning log and update docs (#68681)
### Why?
In https://github.com/vercel/next.js/pull/68622 we fixed a bug that HMR
breaks when `assetPrefix` is set to full URL.
The fix was targeted to the edge case where the user may set `localhost`
to the `assetPrefix`, and technically, when the request is valid the HMR
should work.
However, we do not recommend this pattern as the `assetPrefix` is
intended to be used for setting a CDN URL.
### How?
Therefore we modify the docs to gently imply that the `assetPrefix` is
for CDN URL.
Also, removed the warning log that could be confusing whether we support
it or not.
Setting localhost will still work, but we do not mention about it within
the app.
