Removed an internal detail about route placeholders

Author: javiereguiluz

routing.rst

@@ -466,10 +466,6 @@ a slash. URLs matching this route might look like:
     Symfony provides you with a way to do this by leveraging service container
     parameters. Read more about this in ":doc:`/routing/service_container_parameters`".
 
-.. caution::
-
-    A route placeholder name cannot start with a digit and cannot be longer than 32 characters.
-
 Special Routing Parameters
 ~~~~~~~~~~~~~~~~~~~~~~~~~~