minor #3857 Added little bit information about the route name (WouterJ)

This PR was merged into the 2.3 branch.

Discussion
----------

Added little bit information about the route name

This PR replaces #3263

| Q   | A
| --- | ---
| Doc fix? | no
| New docs? | yes
| Applies to | all
| Fixed tickets | -

Commits
-------

90e654b Applied comments
66d60d2 Update routing.rst

Author: weaverryan

book/routing.rst

@@ -75,7 +75,9 @@ The route is simple:
 The path defined by the ``blog_show`` route acts like ``/blog/*`` where
 the wildcard is given the name ``slug``. For the URL ``/blog/my-blog-post``,
 the ``slug`` variable gets a value of ``my-blog-post``, which is available
-for you to use in your controller (keep reading).
+for you to use in your controller (keep reading). The ``blog_show`` is the
+internal name of the route, which doesn't have any meaning yet and just needs
+to be unique. Later, you'll use it to generate URLs.
 
 The ``_controller`` parameter is a special key that tells Symfony which controller
 should be executed when a URL matches this route. The ``_controller`` string