add comment for IsConsumed

Dapper NET40/SqlMapper.cs

@@ -3891,6 +3891,9 @@ private IEnumerable<T> ReadDeferred<T>(int index, Func<IDataReader, object> dese
             }
             private int gridIndex, readCount;
             private bool consumed;
+            /// <summary>
+            /// Has the underlying reader been consumed?
+            /// </summary>
             public bool IsConsumed
             {
                 get