Revert "[#11375] Removed the workflow_transition_blockers from 4.2 docs" in 4.3

This reverts commit f673d4d.

Author: wouterj

workflow/usage.rst

@@ -588,6 +588,31 @@ transition was blocked::
         }
     }
 
+You can access the message from a Twig template as follows:
+
+.. code-block:: html+twig
+
+    <h2>Publication was blocked because:</h2>
+    <ul>
+        {% for transition in workflow_all_transitions(article) %}
+            {% if not workflow_can(article, transition.name) %}
+                <li>
+                    <strong>{{ transition.name }}</strong>:
+                    <ul>
+                    {% for blocker in workflow_transition_blockers(article, transition.name) %}
+                        <li>
+                            {{ blocker.message }}
+                            {% if blocker.parameters.expression is defined %}
+                                <code>{{ blocker.parameters.expression }}</code>
+                            {% endif %}
+                        </li>
+                    {% endfor %}
+                    </ul>
+                </li>
+            {% endif %}
+        {% endfor %}
+    </ul>
+
 Don't need a human-readable message? You can still use::
 
     $event->setBlocked('true');