Update comment on TryCancel() API

include/grpc++/impl/codegen/server_context.h

@@ -106,14 +106,16 @@ class ServerContext {
   bool IsCancelled() const;
 
   // Cancel the Call from the server. This is a best-effort API and depending on
-  // when this is called, the Call may still appear successful to the client.
+  // when it is called, the RPC may still appear successful to the client.
-  // For example, if called on a separate thread, it might race with the
+  // For example, if TryCancel() is called on a separate thread, it might race
-  // server handler which might return success to the client before TryCancel()
+  // with the server handler which might return success to the client before
-  // was called.
+  // TryCancel() was even started by the thread.
   //
-  // It is the caller's responsibility to prevent such races and ensure that the
+  // It is the caller's responsibility to prevent such races and ensure that if
-  // serverhandler returns Status::CANCELLED if TryCancel() is called (unless
+  // TryCancel() is called, the serverhandler must return Status::CANCELLED. The
-  // the serverhandler is already returning an error code)
+  // only exception is that if the serverhandler is already returning an error
+  // status code, it is ok to not return Status::CANCELLED even if TryCancel()
+  // was called.
   void TryCancel() const;
 
   const std::multimap<grpc::string_ref, grpc::string_ref>& client_metadata() {