add inline docs/explanation for NormalizationForm enum values

Author: Alexander Beedie

src/ast/value.rs

@@ -273,10 +273,18 @@ impl fmt::Display for DateTimeField {
 #[derive(Debug, Clone, PartialEq, Eq, Ord, PartialOrd, Hash)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 #[cfg_attr(feature = "visitor", derive(Visit, VisitMut))]
+/// The Unicode Standard defines four normalization forms, which are intended to eliminate
+/// certain distinctions between visually or functionally identical characters.
+///
+/// See [Unicode Normalization Forms](https://unicode.org/reports/tr15/) for details.
 pub enum NormalizationForm {
+    /// Canonical Decomposition, followed by Canonical Composition.
     NFC,
+    /// Canonical Decomposition.
     NFD,
+    /// Compatibility Decomposition, followed by Canonical Composition.
     NFKC,
+    /// Compatibility Decomposition.
     NFKD,
 }