Document a new public field

src/serialization.rs

@@ -48,6 +48,8 @@ pub struct Encoder {
 /// `Decodable` types to be generated by this decoder. The input is any
 /// arbitrary TOML value.
 pub struct Decoder {
+    /// The TOML value left over after decoding. This can be used to inspect
+    /// whether fields were decoded or not.
     pub toml: Option<Value>,
     cur_field: Option<String>,
 }