Fix Major upgrade article for 2.7.1 changes

Author: wouterj

cookbook/upgrade/major_version.rst

@@ -45,17 +45,19 @@ in your browser, these notices are shown in the web dev toolbar:
 Deprecations in PHPUnit
 ~~~~~~~~~~~~~~~~~~~~~~~
 
-By default, PHPUnit will handle deprecation notices as real errors. This means
-that all tests are aborted because it uses a BC layer.
+By default, deprecations that are triggered by Symfony are silenced. This means
+that, unless you use Xdebug or a custom error handler, your tests will still
+pass without a problem.
 
-To make sure this doesn't happen, you can install the PHPUnit bridge:
+If you want to make sure you don't use any deprecated features, you can install
+the Symfony PHPunit bridge:
 
 .. code-block:: bash
 
     $ composer require symfony/phpunit-bridge
 
-Now, your tests execute normally and a nice summary of the deprecation notices
-is displayed at the end of the test report:
+This bridge registers a custom error handler that will report a nice summary of
+the deprecation notices at the end of the tests:
 
 .. code-block:: text
 
@@ -73,12 +75,36 @@ is displayed at the end of the test report:
         2x in PageAdminTest::testPageList from Symfony\Cmf\SimpleCmsBundle\Tests\WebTest\Admin
         1x in PageAdminTest::testPageEdit from Symfony\Cmf\SimpleCmsBundle\Tests\WebTest\Admin
 
+Now, you can work on fixing these errors. Once you fixed them all, the command
+will end with ``0`` (success) and you're save!
+
+.. sidebar:: Using the Weak Deprecations Mode
+
+    Sometimes, you can't fix all deprecations (e.g. something that is
+    deprecated in 2.6 and you still need to support 2.3). In these cases, you
+    can still use the bridge to fix as many deprecations as possible and then
+    switch to the weak test mode to make your tests pass again. You can do this
+    by using the ``SYMFONY_DEPRECATIONS_HELPER`` env variable:
+
+    .. code-block:: xml
+
+        <!-- phpunit.xml.dist -->
+        <phpunit>
+            <!-- ... -->
+
+            <php>
+                <env name="SYMFONY_DEPRECATIONS_HELPER" value="weak"/>
+            </php>
+        </phpunit>
+
+    (you can also execute the command like ``SYMFONY_DEPRECATIONS_HELPER=weak phpunit``).
+
 .. _upgrade-major-symfony-composer:
 
 2) Update to the New Major Version via Composer
 -----------------------------------------------
 
-If your code is deprecation free, you can update the Symfony library via
+Once your code is deprecation free, you can update the Symfony library via
 Composer by modifying your ``composer.json`` file:
 
 .. code-block:: json
@@ -89,14 +115,14 @@ Composer by modifying your ``composer.json`` file:
         "require": {
             "symfony/symfony": "3.0.*",
         },
-        "...": "...",
+        "...": "..."
     }
 
 Next, use Composer to download new versions of the libraries:
 
 .. code-block:: bash
 
-    $ composer update symfony/symfony
+    $ composer update --with-dependencies symfony/symfony
 
 .. include:: /cookbook/upgrade/_update_all_packages.rst.inc