improve docs

src/csharp/Grpc.Core/IAsyncStreamReader.cs

@@ -44,7 +44,9 @@ namespace Grpc.Core
     /// <para>
     /// <c>MoveNext()</c> operations can be cancelled via a cancellation token. Cancelling
     /// an individual read operation has the same effect as cancelling the entire call
-    /// (which will also result in the read operation returning prematurely).
+    /// (which will also result in the read operation returning prematurely), but the per-read cancellation
+    /// tokens passed to MoveNext() only result in cancelling the call if the read operation haven't finished
+    /// yet.
     /// </para>
     /// </summary>
     /// <typeparam name="T">The message type.</typeparam>