Rules should not overwrite description, but append to it

[alefranz]

See https://github.com/NewDayTechnology/NewDay.Extensions.FunctionsValidationFilter/blob/493af827fc4ba7b87e8086085e2054251f41e841/src/FunctionsValidationFilter/FunctionsValidationDocumentFilter.cs#L90-L91