better document purpose of Identifier

src/common.rs

@@ -105,6 +105,11 @@ impl ListSeparator {
     }
 }
 
+/// In Sass, underscores and hyphens are considered equal
+/// when inside identifiers.
+///
+/// This struct protects that invariant by normalizing all
+/// underscores into hypens.
 #[derive(Debug, Clone, Eq, PartialEq, Hash)]
 pub(crate) struct Identifier(String);