[DOCS][Developer] httpclient: Add an example of lazyness care

docs/developer/src/httpclient.md

@@ -8,4 +8,22 @@ The `$options` are elaborated in [Symfony Doc](https://symfony.com/doc/current/h
 Please note that the HTTP Client is [lazy](https://en.wikipedia.org/wiki/Lazy_evaluation),
 which makes it very important to bear in mind [Network Errors](https://symfony.com/doc/current/http_client.html#dealing-with-network-errors),
 
+An example where this behaviour has to be considered:
+
+```php
+if (Common::isValidHttpUrl($url)) {
+    $head = HTTPClient::head($url);
+    // This must come before getInfo given that Symfony HTTPClient is lazy (thus forcing curl exec)
+    try {
+        $headers = $head->getHeaders();
+    } catch (ClientException $e) {
+        throw new InvalidArgumentException(previous: $e);
+    }
+    $url      = $head->getInfo('url'); // The last effective url (after getHeaders, so it follows redirects)
+    $url_hash = hash(self::URLHASH_ALGO, $url);
+} else {
+    throw new InvalidArgumentException();
+}
+```
+
 What you can take from Responses is specified [here](https://symfony.com/doc/current/http_client.html#processing-responses).