add docs to the err var

Author: 0xNeshi

src/error.rs

@@ -37,6 +37,10 @@ pub enum ScannerError {
     #[error("Subscription closed")]
     SubscriptionClosed,
 
+    /// A subscription consumer could not keep up and some internal messages were skipped.
+    ///
+    /// The contained value is the number of skipped messages reported by the underlying channel.
+    /// After emitting this error, the subscription stream may continue with newer items.
     #[error("Subscription lagged")]
     Lagged(u64),
 }