Add documentation comment for nothing type property

Author: mattt

Sources/PackageModel/Product.swift

@@ -89,6 +89,7 @@ public enum ProductFilter: Equatable, Hashable {
     /// A set of specific products requested by one or more client packages.
     case specific(Set<String>)
 
+    /// No products, targets, or tests are requested.
     public static var nothing: ProductFilter { .specific([]) }
 
     public func union(_ other: ProductFilter) -> ProductFilter {