[Serializer] Add deprecation missing from UPGRADE files

UPGRADE-3.2.md

@@ -116,6 +116,11 @@ HttpFoundation
      - `isInvalid`/`isSuccessful`/`isRedirection`/`isClientError`/`isServerError`
      - `isOk`/`isForbidden`/`isNotFound`/`isRedirect`/`isEmpty`
 
+Serializer
+----------
+
+ * Method `AbstractNormalizer::instantiateObject()` will have a 6th `$format = null` argument in Symfony 4.0. Not defining it when overriding the method is deprecated.
+
 TwigBridge
 ----------
 

UPGRADE-4.0.md

@@ -170,6 +170,8 @@ Serializer
  * The ability to pass a Doctrine `Cache` instance to the `ClassMetadataFactory`
    class has been removed. You should use the `CacheClassMetadataFactory` class
    instead.
+   
+ * Not defining the 6th argument `$format = null` of the `AbstractNormalizer::instantiateObject()` method when overriding it is not supported anymore.
 
 Translation
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