Clarify the concurrency of OnDone with respect to the reactor, and ca…

…ll out explicitly that outside use of the reactor might need holds in order for OnDone to be called concurrently.

PiperOrigin-RevId: 703460223

include/grpcpp/support/client_callback.h

@@ -316,6 +316,10 @@ class ClientBidiReactor : public internal::ClientReactor {
   /// all cases. If it is not called, it indicates an application-level problem
   /// (like failure to remove a hold).
   ///
+  /// OnDone is called exactly once, and not concurrently with any (other)
+  /// reaction. (Holds may be needed (see above) to prevent OnDone from being
+  /// called concurrently with calls to the reactor from outside of reactions.)
+  ///
   /// \param[in] s The status outcome of this RPC
   void OnDone(const grpc::Status& /*s*/) override {}