minor #1382 Included extend() explanation in services documentation chapter (hvt)

This PR was squashed before being merged into the 2.2.x-dev branch (closes #1382).

Discussion
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Included extend() explanation in services documentation chapter

Together with the Twig add global and filter example.

Commits
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65d6d344 One change too many
124654fc Some typo fixes
7d1e96c5 Included extend() explanation in services documentation chapter

doc/services.rst

@@ -166,6 +166,31 @@ However, you can inject it via `use($app)`::
         // ...
     });
 
+Modify services after definition
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Sometimes you want to alter a service after its definition. Pimple facilitates
+this by extending the already defined service.
+
+First argument of the ``extend`` method is the name of the service you want to
+modify. Second argument is a callable. This callable is executed with the service
+you want to alter as its first argument, the service container itself is provided
+in the second argument.
+
+.. note::
+
+    Be sure to return the modified service in the callable.
+
+You can use this pattern to add functionality to :doc:Twig <providers/twig> for
+example::
+
+    $app->extend('twig', function($twig, $app) {
+        $twig->addGlobal('pi', 3.14);
+        $twig->addFilter('levenshtein', new \Twig_Filter_Function('levenshtein'));
+        
+        return $twig;
+    });
+
 Core services
 -------------