add comment for explicit in the Preserve enum

src/uu/cp/src/cp.rs

@@ -184,6 +184,8 @@ pub struct Attributes {
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum Preserve {
+    // explicit means is the --no-preserve flag is used or not to distinguish out the default value.
+    // e.g. --no-preserve=mode means mode = No { explicit = true }
     No { explicit: bool },
     Yes { required: bool },
 }