minor #18442 [HttpFoundation] Improve phpdoc (mnapoli)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #18442).

Discussion
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[HttpFoundation] Improve phpdoc

| Q             | A
| ------------- | ---
| Branch?       | master?
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

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()`.

Commits
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6bfbb2e [HttpFoundation] Improve phpdoc

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