minor #21773 [Uid] Clarify Doctrine UUID generator usage and configuration (lacatoire)

This PR was submitted for the 8.1 branch but it was merged into the 6.4 branch instead.

Discussion
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 [Uid] Clarify Doctrine UUID generator usage and configuration

Clarify the difference between using UuidGenerator::class and the doctrine.uuid_generator service when storing UUIDs with Doctrine, and document how to properly use the configured UUID version.
https://github.com/symfony/symfony-docs/issues/21528

Commits
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742ce13d0 docs(uid): clarify Doctrine UUID generator usage

components/uid.rst

@@ -350,6 +350,19 @@ entity primary keys::
 
         // ...
     }
+.. note::
+
+    When using DoctrineBundle, using ``UuidGenerator::class`` creates a new
+    generator instance and does **not** use the ``doctrine.uuid_generator``
+    service. As a result, the UUID version configured via the FrameworkBundle
+    (for example UUIDv6 or UUIDv7) is ignored.
+
+When working with DoctrineBundle, prefer using the generator service instead::
+
+    #[ORM\Id]
+    #[ORM\Column(type: 'uuid', unique: true)]
+    #[ORM\GeneratedValue(strategy: 'CUSTOM')]
+    #[ORM\CustomIdGenerator('doctrine.uuid_generator')]
 
 .. warning::