minor #10113 Problem with transactions (betzholz)

This PR was submitted for the 4.1 branch but it was merged into the 4.0 branch instead (closes #10113).

Discussion
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Problem with transactions

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Commits
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12c9562 Problem with transactions

Author: javiereguiluz

doctrine/pdo_session_storage.rst

@@ -29,6 +29,8 @@ To use it, first register a new handler service:
                     # If you're using Doctrine & want to re-use that connection, then:
                     # comment-out the above 2 lines and uncomment the line below
                     # - !service { class: PDO, factory: 'database_connection:getWrappedConnection' }
+                    # sometimes you get transaction problems (ex. after login) then add
+                    # - { lock_mode: 1 }
 
     .. code-block:: xml