Document the Requirement constants in routing.rst

Author: nicwortel

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