docs: mention request header filtering on migration

Author: ousamabenyounes

docs/migrate.md

@@ -23,7 +23,7 @@ This guide covers a basic migration for a typical PHP application.
 ## HTTP request filtering
 
 When migrating from a stock Nginx or Apache package, check any request filtering that was done by the web server before PHP-FPM received the request.
-FrankenPHP is built on Caddy, so valid HTTP methods and headers are passed through Caddy's normal routing unless your `Caddyfile` rejects them first.
+FrankenPHP is built on Caddy, so valid HTTP methods and headers are passed through Caddy's normal routing unless your `Caddyfile` or another proxy rejects them first.
 
 For example, some web servers reject `TRACE` requests by default.
 If your application should keep that behavior, add an explicit matcher before `php_server`:
@@ -39,6 +39,7 @@ example.com {
 ```
 
 Apply the same approach to any stricter method or header policy your previous front web server enforced.
+This is also relevant for request header names: some front web servers or proxies reject or drop valid-but-uncommon HTTP field names before PHP-FPM sees them, while FrankenPHP may pass them to the PHP application.
 
 ## Step 1: replace your web server config