minor #7315 [Workflow] Add tip for workflow configuration default values (dominikhajduk)

This PR was squashed before being merged into the 3.2 branch (closes #7315).

Discussion
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[Workflow] Add tip for workflow configuration default values

Added tip for http://symfony.com/doc/current/workflow/usage.html configuration.

Tested and based on great examples provided by @lyrixx in: https://github.com/lyrixx/SFLive-Paris2016-Workflow/blob/master/app/config/workflow.yml#L3 and
https://github.com/lyrixx/SFLive-Paris2016-Workflow/blob/master/src/AppBundle/Entity/Article.php#L24

Commits
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6262d24 [Workflow] Add tip for workflow configuration default values

Author: xabbuh

workflow/usage.rst

@@ -148,6 +148,12 @@ like this:
     The marking store type could be "multiple_state" or "single_state".
     A single state marking store does not support a model being on multiple places
     at the same time.
+    
+.. tip::
+
+    The ``type`` (default value ``single_state``) and ``arguments`` (default value ``marking``)
+    attributes of the ``marking_store`` option are optional. If omitted, their default values
+    will be used.
 
 With this workflow named ``blog_publishing``, you can get help to decide
 what actions that are allowed on a blog post. ::