why use different behavior for the description of query and mutation

* **What is the current behavior?**
Now, the query description only use the `description=...` args for docs, the description or the __doc__ in Typeclass only use for Type docs.
but the mutation use the `description` args or the description of mutation class or the __doc__ of mutation class for both mutation docs and mutation class docs.

* **What is the expected behavior?**
Both query and mutation should share the same logic for docs, first use the `description` arg of the field, then the description of meta class of class, the last is using __doc__ of (the type or mutation) class.

* **Other information** (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow)
dig into code, I find the graphene.mutation has a classmethod func named Field which return graphene.Field with `description=description or cls._meta.description`.

Commencez par comparer la gestion de la description du champ query avec la méthode de classe Field de graphene.mutation, qui renvoie un graphene.Field en utilisant description ou cls._meta.description. Suivez la manière dont __doc__ et les descriptions de type ou de mutation parviennent à la documentation générée. Le travail est considéré comme terminé lorsque les champs query et mutation suivent la même priorité documentée et que des tests confirment les descriptions obtenues.

Rédigé par le modèle d'indexation à partir du texte de l'issue.