provide more complete description of typed string

Author: maxcountryman

src/ast/mod.rs

@@ -205,7 +205,9 @@ pub enum Expr {
     Nested(Box<Expr>),
     /// A literal value, such as string, number, date or NULL
     Value(Value),
-    /// A structure containing a `String` as well as an associated data type.
+    /// A constant of form `<data_type> 'value'`.
+    /// This can represent ANSI SQL `DATE`, `TIME`, and `TIMESTAMP` literals (such as `DATE '2020-01-01'`),
+    /// as well as constants of other types (a non-standard PostgreSQL extension).
     TypedString { data_type: DataType, value: String },
     /// Scalar function call e.g. `LEFT(foo, 5)`
     Function(Function),