docs: updated docs to reflect host/port setting changes

docs/next/en-US/updating.md

@@ -8,7 +8,8 @@ Perseus v0.3.0 added significant architectural changes to Perseus under the hood
 4. Remove any custom config managers you may have, they've been replaced by [mutable and immutable stores](:stores).
 5. Take anything Perseus-related wrapped in `Rc::new` (these will be all through your template definitions and error pages) and remove the `Rc::new`, Perseus now handles that internally, with no performance cost!
 6. If you're using i18n, add the `translator-fluent` flag to `perseus` in your `Cargo.toml`. If you're not, your Wasm bundle size has been reduced!
-7. Update your code for the remaining breaking changes listed in [the CHANGELOG](https://github.com/arctic-hen7/perseus/blob/main/CHANGELOG).
+7. Change any uses of the `HOST` and `PORT` environment variables to the `--host` and `--port` flags on `perseus serve` in development, and use `PERSEUS_HOST` and `PERSEUS_PORT` in production. The original environment variable names will still work in deployments, but they'll be deprecated in v0.4.0. 
+8. Update your code for the remaining breaking changes listed in [the CHANGELOG](https://github.com/arctic-hen7/perseus/blob/main/CHANGELOG).
 
 Perseus v0.3.0 also changed a few common idioms, like breaking out the `.template()` call into a separate function `template_fn()`. This is no longer recommended, though it will still work fine. You can check out the [examples directory](https://github.com/arctic-hen7/perseus/tree/main/examples) to see how things are a bit nicer now in terms of formatting.