Fix serde docs

tokio-postgres/src/types/serde_json_1.rs

@@ -6,6 +6,7 @@ use std::io::Read;
 
 use crate::types::{FromSql, IsNull, ToSql, Type};
 
+/// A wrapper type to allow arbitrary `Serialize`/`Deserialize` types to convert to Postgres JSON values.
 #[derive(Debug)]
 pub struct Json<T>(pub T);