docs: Improve collection_code/key TODO comment

Author: kdmccormick

openedx/core/djangoapps/content_libraries/rest_api/serializers.py

@@ -285,6 +285,7 @@ class ContentLibraryCollectionSerializer(serializers.ModelSerializer):
     Serializer for a Content Library Collection
     """
     # Expose Collection.collection_code as "key" to preserve the REST API field name.
+    # https://github.com/openedx/openedx-platform/issues/38406
     key = serializers.CharField(source='collection_code')
 
     class Meta: