core template field-multiple-value-form do not use correct class for description

Problem/Motivation

All the descriptions of the components are using the .form-item__description class and the .description class as well I think.
But when you have a multi-value field (let say illimited reference item), the description of this field doesn't get this class.

NOW:

SHOULD BE:

Proposed resolution

Add the class through preprocess or in the the template
OR
Remove this class everywhere and use only the description class (and move the css rules on this class).
Question: Is there any good reason to only have css style for .form-item__description and not for .description