feature #5336 Adding a paragraph about updating multiple packages during an update (weaverryan)

This PR was submitted for the 2.7 branch but it was merged into the 2.3 branch instead (closes #5336).

Discussion
----------

Adding a paragraph about updating multiple packages during an update

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | n/a

Hi guys!

For my 2.7 upgrade, I needed to run `composer update symfony/symfony twig/twig`. I'm trying to include a note to help other people. I'd appreciate opinions on wording :).

Thanks!

Commits
-------

bb0acd0 fixing bad wording
8a30d14 using --with-dependencies
7a28919 Adding a paragraph about updating multiple packages at once during an upgrade

Author: weaverryan

cookbook/upgrade/_update_all_packages.rst.inc

@@ -1,3 +1,6 @@
+Upgrading other Packages
+~~~~~~~~~~~~~~~~~~~~~~~~
+
 You may also want to upgrade the rest of your libraries. If you've done a
 good job with your `version constraints`_ in ``composer.json``, you can do
 this safely by running:

cookbook/upgrade/_update_dep_errors.rst.inc

@@ -0,0 +1,19 @@
+Dependency Errors
+~~~~~~~~~~~~~~~~~
+
+If you get a dependency error, it may simply mean that you need to upgrade
+other Symfony dependencies too. In that case, try the following command:
+
+    $ composer update symfony/symfony --with-dependencies
+
+This updates ``symfony/symfony`` and *all* packages that it depends on, which will
+include several other packages. By using tight version constraints in
+``composer.json``, you can control what versions each library upgrades to.
+
+If this still doesn't work, your ``composer.json`` file may specify a version
+for a library that is not compatible with the newer Symfony version. In that
+case, updating that library to a newer version in ``composer.json`` may solve
+the issue.
+
+Or, you may have deeper issues where different libraries depend on conflicting
+versions of other libraries. Check your error message to debug.

cookbook/upgrade/major_version.rst

@@ -98,6 +98,8 @@ Next, use Composer to download new versions of the libraries:
 
     $ composer update symfony/symfony
 
+.. include:: /cookbook/upgrade/_update_dep_errors.rst.inc
+
 .. include:: /cookbook/upgrade/_update_all_packages.rst.inc
 
 .. _upgrade-major-symfony-after:

cookbook/upgrade/minor_version.rst

@@ -41,6 +41,8 @@ Next, use Composer to download new versions of the libraries:
 
     $ composer update symfony/symfony
 
+.. include:: /cookbook/upgrade/_update_dep_errors.rst.inc
+
 .. include:: /cookbook/upgrade/_update_all_packages.rst.inc
 
 .. _`upgrade-minor-symfony-code`: