Add a comment in fr.js

cf: https://github.com/ElemeFE/element/pull/9053

Add a comment to explain why year must be empty, because we definitely will not remember that!
(I am the one who filled it the last time :/)

src/locale/lang/fr.js

@@ -20,7 +20,7 @@ export default {
       nextYear: 'Année suivante',
       prevMonth: 'Mois précédent',
       nextMonth: 'Mois suivant',
-      year: '',
+      year: '', // In french, like in english, we don't say "Année" after the year number.
       month1: 'Janvier',
       month2: 'Février',
       month3: 'Mars',