feature symfony#3368 The host parameter has to be in defaults, not requirements (MarieMinasyan)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes symfony#3368).

Discussion
----------

The host parameter has to be in defaults, not requirements

 Q             | A
 ------------- | ---
 Doc fix?      | yes
 New docs?     | no
 Applies to    | 2.2+

Also has to be applied to 2.2 & 2.3 branches

Commits
-------

8a1afae Moved tip block to remove confusion
9c30509 The host parameter has to be in defaults, not requirements

Author: weaverryan

components/routing/hostname_pattern.rst

@@ -176,25 +176,27 @@ instance, if you want to match both ``m.example.com`` and
 
         return $collection;
 
-.. tip::
-
-    Make sure you also include a default option for the ``subdomain``
-    placeholder, otherwise you need to include the subdomains value each time
-    you generate the route.
-
 .. sidebar:: Using Service Parameters
 
     You can also use service parameters if you do not want to hardcode the
     hostname:
 
+    .. tip::
+
+       Make sure you also include a default option for the ``subdomain``
+       placeholder, otherwise you need to include the subdomains value each time
+       you generate the route.
+
     .. configuration-block::
 
         .. code-block:: yaml
 
             mobile_homepage:
                 path:     /
                 host:     "m.{domain}"
-                defaults: { _controller: AcmeDemoBundle:Main:mobileHomepage }
+                defaults:
+                    _controller: AcmeDemoBundle:Main:mobileHomepage
+                    domain: "%domain%"
                 requirements:
                     domain: "%domain%"
 
@@ -212,6 +214,7 @@ instance, if you want to match both ``m.example.com`` and
 
                 <route id="mobile_homepage" path="/" host="m.example.com">
                     <default key="_controller">AcmeDemoBundle:Main:mobileHomepage</default>
+                    <default key="domain">%domain%</requirement>
                     <requirement key="domain">%domain%</requirement>
                 </route>
 
@@ -228,6 +231,7 @@ instance, if you want to match both ``m.example.com`` and
             $collection = new RouteCollection();
             $collection->add('mobile_homepage', new Route('/', array(
                 '_controller' => 'AcmeDemoBundle:Main:mobileHomepage',
+                'domain' => '%domain%',
             ), array(
                 'domain' => '%domain%',
             ), array(), 'm.{domain}'));