minor #12481 Documented the doctrine_clear_entity_manager middleware (javiereguiluz)

This PR was merged into the 4.4 branch.

Discussion
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Documented the doctrine_clear_entity_manager middleware

Fixes #11657.

Commits
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53fb21d Documented the doctrine_clear_entity_manager middleware

Author: javiereguiluz

messenger.rst

@@ -1407,6 +1407,11 @@ may want to use:
                             # or pass a different entity manager to any
                             #- doctrine_transaction: ['custom']
 
+                            # After handling, the clear() method is executed in the
+                            # entity manager, which reduces the memory consumption and
+                            # avoids side-effects related to unrefreshed entities
+                            - doctrine_clear_entity_manager
+
     .. code-block:: xml
 
         <!-- config/packages/messenger.xml -->
@@ -1432,6 +1437,8 @@ may want to use:
                             <framework:argument>custom</framework:argument>
                         </framework:middleware>
                         -->
+
+                        <framework:middleware id="doctrine_clear_entity_manager"/>
                     </framework:bus>
                 </framework:messenger>
             </framework:config>
@@ -1450,12 +1457,17 @@ may want to use:
                             'doctrine_close_connection',
                             // Using another entity manager
                             ['id' => 'doctrine_transaction', 'arguments' => ['custom']],
+                            'doctrine_clear_entity_manager',
                         ],
                     ],
                 ],
             ],
         ]);
 
+.. versionadded:: 4.4
+
+    The ``doctrine_clear_entity_manager`` middleware was introduced in Symfony 4.4.
+
 Messenger Events
 ~~~~~~~~~~~~~~~~