[13.x] Document the trustedproxy.proxies configuration fallback for trusted proxies

[hohanov]

The Configuring Trusted Proxies section only explains how to set trusted proxies with $middleware->trustProxies(at: ...) in bootstrap/app.php.

It does not mention that Illuminate\Http\Middleware\TrustProxies falls back to the trustedproxy.proxies configuration value when no proxies were set that way:

$trustedIps = $this->proxies() ?: config('trustedproxy.proxies');

framework/src/Illuminate/Http/Middleware/TrustProxies.php

Line 69 in 1de9acf

$trustedIps = $this->proxies() ?: config('trustedproxy.proxies');

This fallback was restored in #41322 ("like back in the days when we had Fideloper's package"), but it was never documented. An earlier docs PR, laravel/docs#7457, was closed without being merged.

Why it should be documented

• Proxy addresses usually differ between environments: none locally, a load balancer subnet in staging and production. The withMiddleware callback in bootstrap/app.php runs before the environment and configuration are loaded. Because of that, config() cannot be used there, and env() returns null for values from .env (see [11.x] Add trustedproxy config file to allow dynamic assignment for IPs #52085). Unlike trustHosts(), trustProxies(at:) does not accept a closure.
• The configuration fallback is the only built-in way to set trusted proxies per environment, and it works with config:cache.

Proposed addition

After "Trusting All Proxies" in requests.md:

#### Configuring Trusted Proxies via Configuration

If you do not specify trusted proxies using the `trustProxies` method,
the `TrustProxies` middleware will use the `trustedproxy.proxies`
configuration value instead. This is useful when your trusted proxies
differ between environments, since configuration values are not yet
available when `bootstrap/app.php` is evaluated. To get started, create
a `config/trustedproxy.php` configuration file:

    <?php

    return [
        'proxies' => env('TRUSTED_PROXIES'),
    ];

The value may be `*`, an array of IP addresses or CIDR ranges, or a
comma-separated string:

    TRUSTED_PROXIES=192.168.1.1,10.0.0.0/8