[HttpFoundation] Improve phpdoc

Improve the phpdoc for the `$default` parameter of the `get()` method. It wasn't clear when the default value would be used (whether the key would not exist or the value was `null` or nullish).

The comment is now in sync with `Symfony\Component\HttpFoundation\ParameterBag::get()`.

src/Symfony/Component/HttpFoundation/Request.php

@@ -674,7 +674,7 @@ class Request
      * public property instead (query, attributes, request).
      *
      * @param string $key     the key
-     * @param mixed  $default the default value
+     * @param mixed  $default the default value if the parameter key does not exist
      * @param bool   $deep    is parameter deep in multidimensional array
      *
      * @return mixed