fix(performance): add warning about Serializer groups being mandatory in order to trigger EagerLoadingExtension (#1814)

Co-authored-by: Gaëtan Petit <[email protected]>

Author: gaetan-petit

core/performance.md

@@ -199,6 +199,8 @@ public $foo;
 ...
 ```
 
+> **Warning**: in order to trigger the `EagerLoadingExtension` you must use [Serializer groups](serialization.md) on relations properties.
+
 #### Max Joins
 
 There is a default restriction with this feature. We allow up to 30 joins per query. Beyond that, an