docs: document HTTP request filtering differences

[ousamabenyounes]

Fix #2521.

Changes

• Document request-filtering differences to check when migrating from Nginx or Apache plus PHP-FPM.
• Add a TRACE method example showing how to reject it explicitly in the Caddyfile before php_server.
• Mention that stricter header-name filtering may also need to be enforced by the Caddyfile or another proxy when migrating.
• Keep FrankenPHP's default behavior unchanged.

Validation

Verified with Docker using dunglas/frankenphp:1.12.6-php8.5 and a PHP script that prints the request method and incoming HTTP_X_* server variables.

Default Caddyfile:

GET /index.php      -> 200, method=GET
TRACE /index.php    -> 200, method=TRACE
special header name -> passed to PHP as HTTP_X_SPECIAL!#$%&*+_^|~

With the documented matcher:

GET /index.php      -> 200, method=GET
TRACE /index.php    -> 405
special header name -> still passed to PHP unless filtered separately

Also ran git diff --check.