minor #16926 [Routing] Document the Requirement constants in routing.rst (nicwortel)

This PR was merged into the 6.1 branch.

Discussion
----------

[Routing] Document the Requirement constants in routing.rst

Fixes #16718

symfony/symfony#45528 by `@fancyweb` introduced the `Symfony\Component\Routing\Requirement\Requirement` enum in Symfony 6.1, as a solution for symfony/symfony#26524.

It was blogged about in https://symfony.com/blog/new-in-symfony-6-1-improved-routing-requirements-and-utf-8-parameters, but I couldn't find it anywhere in the Routing documentation.

This PR adds a small hint about the use of the `Requirement` enum for common route requirements:

![The Requirement enum contains a collection of commonly used regular-expression constants such as digits, dates and UUIDs which can be used as route parameter requirements.](https://user-images.githubusercontent.com/1055691/176497123-24b9bf6e-1b79-4385-baee-0eb132ae8944.png)

I'm open for suggestions to further improve this. I decided to use a small info block instead without code examples, because it doesn't seem to be a big feature and I'm not sure how well it is supported across all configuration formats. Let me know if you would like to see some code examples.

Commits
-------

571647d Document the Requirement constants in routing.rst

Author: javiereguiluz

routing.rst

@@ -772,6 +772,12 @@ URL                       Route          Parameters
 ``/blog/my-first-post``   ``blog_show``  ``$slug`` = ``my-first-post``
 ========================  =============  ===============================
 
+.. tip::
+
+    The :class:`Symfony\\Component\\Routing\\Requirement\\Requirement` enum
+    contains a collection of commonly used regular-expression constants such as
+    digits, dates and UUIDs which can be used as route parameter requirements.
+
 .. tip::
 
     Route requirements (and route paths too) can include