Cannot set validUntil on remark

[raffazizzi]

Setting @validUntil on <remark> causes the error:

assertion failed: A deprecated construct should include, whenever            possible, an explanation, but this remarks does not have a child            <desc type ="deprecationInfo" >

... but <remark> cannot have a child <desc>. This should probably be a warning, or it should be removed from elements that cannot contain <desc>.