[set] Add missing comments (#175)

set/map.go

@@ -2,9 +2,11 @@ package set
 
 import "github.com/moorara/algo/generic"
 
-// Mapper
+// Mapper is a function for converting a member of a set from one type to another type.
 type Mapper[T, U any] func(T) U
 
+// Map converts a set from one type to another type.
+// You need to provide a compare function for the new type.
 func (f Mapper[T, U]) Map(s Set[T], equal generic.EqualFunc[U]) Set[U] {
 	members := make([]U, 0)
 	for _, m := range s.Members() {