Add note on hijacking

include/grpcpp/impl/codegen/interceptor.h

@@ -56,8 +56,10 @@ enum class InterceptionHookPoints {
   POST_RECV_MESSAGE,
   POST_RECV_STATUS /* client only */,
   POST_RECV_CLOSE /* server only */,
-  /* This is a special hook point available to both clients and servers. It is
-     illegal for an interceptor to block/delay this operation */
+  /* This is a special hook point available to both clients and servers when
+     TryCancel() is performed. It is illegal for an interceptor to block/delay
+     this operation. ALL interceptors see this hook point irrespective of
+     whether the RPC was hijacked or not. */
   PRE_SEND_CANCEL,
   NUM_INTERCEPTION_HOOKS
 };
@@ -71,7 +73,7 @@ class InterceptorBatchMethods {
   // Calling this will signal that the interceptor is done intercepting the
   // current batch of the RPC.
   // Proceed is a no-op if the batch contains PRE_SEND_CANCEL. Simply returning
-  // from the Intercept method does the job of continuing the RPC.
+  // from the Intercept method does the job of continuing the RPC in this case.
   virtual void Proceed() = 0;
   // Calling this indicates that the interceptor has hijacked the RPC (only
   // valid if the batch contains send_initial_metadata on the client side)