chore: explain double validation

Author: jesperhodge

src/openedx_tagging/rest_api/v1/serializers.py

@@ -220,8 +220,10 @@ class ObjectTagUpdateBodySerializer(serializers.Serializer):  # pylint: disable=
 
 def validate_tag_value(value, context, original_value=None):
     """
-    Validate this tag value is unique within the current taxonomy context and
-    does not contain forbidden characters.
+    Validates the incoming request early:
+    - This tag is unique, not a duplicate. (The model does not validate this sufficiently.)
+    - There are no forbidden / reserved characters present. There is an additional model-side validation for this as well,
+    but we are keeping this so we can validate the incoming request immediately.
     """
     taxonomy_id = context.get("taxonomy_id")
     original_tag = Tag.objects.filter(taxonomy_id=taxonomy_id, value=original_value).first() if original_value else None