minor #4432 [Reference][Twig] tweaks to the Twig reference (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

[Reference][Twig] tweaks to the Twig reference

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

Commits
-------

420d411 tweaks to the Twig reference

reference/twig_reference.rst

@@ -7,15 +7,19 @@ Symfony Twig Extensions
 =======================
 
 Twig is the default template engine for Symfony. By itself, it already contains
-a lot of built-in functions, filters, tags and tests (`http://twig.sensiolabs.org/documentation`_
-then scroll to the bottom).
+a lot of built-in functions, filters, tags and tests (learn more about them
+from the the `Twig Reference`_).
 
-Symfony adds more custom extension on top of Twig to integrate some components
-into the Twig templates. Below is information about all the custom functions,
-filters, tags and tests that are added when using the Symfony Core Framework.
+Symfony adds more custom extensions on top of Twig to integrate some components
+into the Twig templates. You can find more information about the custom
+:ref:`functions <reference-twig-functions>`, :ref:`filters <reference-twig-filters>`,
+:ref:`tags <reference-twig-tags>` and :ref:`tests <reference-twig-tests>`
+that are added when using the Symfony Core Framework.
 
 There may also be tags in bundles you use that aren't listed here.
 
+.. _reference-twig-functions:
+
 Functions
 ---------
 
@@ -40,7 +44,7 @@ render
 Renders the fragment for the given controller (using the `controller`_ function)
 or URI. For more information, see :ref:`templating-embedding-controller`.
 
-The render strategy can be specified in the ``strategy`` key of the options. 
+The render strategy can be specified in the ``strategy`` key of the options.
 
 .. tip::
 
@@ -65,7 +69,7 @@ Generates an ESI tag when possible or falls back to the behaviour of
 .. tip::
 
     The URI can be generated by other functions, like `path`_ and `url`_.
-    
+
 .. tip::
 
     The ``render_esi()`` function is an example of the shortcut functions
@@ -354,6 +358,8 @@ Returns the absolute URL (with scheme and host) for the given route. If
 ``schemeRelative`` is enabled, it'll create a scheme-relative URL. More
 information in :ref:`book-templating-pages`.
 
+.. _reference-twig-filters:
+
 Filters
 -------
 
@@ -554,6 +560,8 @@ file_link
 Generates a link to the provided file (and optionally line number) using a
 preconfigured scheme.
 
+.. _reference-twig-tags:
+
 Tags
 ----
 
@@ -620,6 +628,8 @@ trans_default_domain
 
 This will set the default domain in the current template.
 
+.. _reference-twig-tests:
+
 Tests
 -----
 
@@ -669,5 +679,5 @@ Those bundles can have other Twig extensions:
   ``{% image %}`` tags. You can read more about them in
   :doc:`the Assetic Documentation </cookbook/assetic/asset_management>`.
 
+.. _`Twig Reference`: http://twig.sensiolabs.org/documentation#reference
 .. _`the official Twig Extensions documentation`: http://twig.sensiolabs.org/doc/extensions/index.html
-.. _`http://twig.sensiolabs.org/documentation`: http://twig.sensiolabs.org/documentation