Tweak comment.

rest_framework/fields.py

@@ -187,9 +187,8 @@ class WritableField(Field):
                 native = data[field_name]
         except KeyError:
             if self.default is not None and not self.root.partial:
+                # Note: partial updates shouldn't set defaults
                 native = self.default
-                # partial serializers shouldn't set the default field to avoid
-                # overriding the previously set value
             else:
                 if self.required:
                     raise ValidationError(self.error_messages['required'])