[#6894] Adding link to Doctrine

Author: weaverryan

doctrine.rst

@@ -558,8 +558,7 @@ Take a look at the previous example in more detail:
 .. note::
 
     If the ``flush()`` call fails, a ``Doctrine\ORM\ORMException`` exception
-    is thrown, which you can catch by wrapping the ``flush()`` method in a
-    ``try...catch`` block.
+    is thrown. See `Transactions and Concurrency`_.
 
 Whether creating or updating objects, the workflow is always the same. In
 the next section, you'll see how Doctrine is smart enough to automatically
@@ -877,3 +876,4 @@ Learn more
 .. _`DoctrineFixturesBundle`: https://symfony.com/doc/current/bundles/DoctrineFixturesBundle/index.html
 .. _`FrameworkExtraBundle documentation`: https://symfony.com/doc/current/bundles/SensioFrameworkExtraBundle/annotations/converters.html
 .. _`newer utf8mb4 character set`: https://dev.mysql.com/doc/refman/5.5/en/charset-unicode-utf8mb4.html
+.. _`Transactions and Concurrency`: http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/transactions-and-concurrency.html