feature #6783 Routing: add explanation for the "_fragment" parameter (alexislefebvre)

This PR was squashed before being merged into the master branch (closes #6783).

Discussion
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Routing: add explanation for the "_fragment" parameter

Add explanation about this PR: symfony/symfony#12979

Presented in http://symfony.com/blog/new-in-symfony-3-2-routing-improvements

I copy-pasted the start of the announcement as the explanation since I couldn't have found a better explanation. I hope that won't be a problem.

Commits
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d657abd Routing: add explanation for the "_fragment" parameter

Author: wouterj

routing.rst

@@ -470,7 +470,7 @@ Special Routing Parameters
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 As you've seen, each routing parameter or default value is eventually available
-as an argument in the controller method. Additionally, there are three parameters
+as an argument in the controller method. Additionally, there are four parameters
 that are special: each adds a unique piece of functionality inside your application:
 
 ``_controller``
@@ -480,6 +480,14 @@ that are special: each adds a unique piece of functionality inside your applicat
 ``_format``
     Used to set the request format (:ref:`read more <routing-format-param>`).
 
+``_fragment``
+
+ .. versionadded:: 3.2
+    The ``_fragment`` parameter was introduced in Symfony 3.2.
+
+    Used to set the fragment identifier, the optional last part of a URL that
+    starts with a ``#`` character and is used to identify a portion of a document.
+
 ``_locale``
     Used to set the locale on the request (:ref:`read more <translation-locale-url>`).