minor #8213 [Workflow] Document the new transition completed event (izzyp)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes #8213).

Discussion
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[Workflow] Document the new transition completed event

as per PR - symfony/symfony#22587

Commits
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b8249ab [Workflow] Document the new transition completed event

Author: javiereguiluz

workflow/usage.rst

@@ -240,6 +240,16 @@ order:
     * ``workflow.entered``
     * ``workflow.[workflow name].entered``
     * ``workflow.[workflow name].entered.[place name]``
+    
+``workflow.completed``
+    The object has completed this transition.
+    
+    The three events being dispatched are:
+
+    * ``workflow.transition``
+    * ``workflow.[workflow name].completed``
+    * ``workflow.[workflow name].completed.[transition name]``
+
 
 ``workflow.announce``
     Triggered for each transition that now is accessible for the object.