cubicweb #2730282 Update the doc for translation wrt ValidationError handling [open]

We see currently:

msg = self._cw._('....')
raise ValidationError(... {...: msg})

Should be:

msg = _('....')
raise ValidationError(... {...: msg})
prioritynormal
typebug
done in<not specified>
load0.200
load left0.200
closed by<not specified>