Fixup for https://phabricator.services.mozilla.com/D233126

Signed-off-by: Oriol Brufau <[email protected]>

style/values/computed/length_percentage.rs

@@ -217,8 +217,11 @@ pub enum Unpacked<'a> {
 
 /// An unpacked `<length-percentage>` that mutably borrows the `calc()` variant.
 enum UnpackedMut<'a> {
+    /// A `calc()` value
     Calc(&'a mut CalcLengthPercentage),
+    /// A length value
     Length(Length),
+    /// A percentage value
     Percentage(Percentage),
 }