Add note on the experimental nature of the additional method in ServerInterface

include/grpcpp/impl/codegen/server_interface.h

@@ -333,7 +333,12 @@ class ServerInterface : public internal::CallHook {
   }
 
  private:
-  virtual const std::vector<
+  // EXPERIMENTAL
+  // Getter method for the vector of interceptor factory objects.
+  // Returns a nullptr (rather than being pure) since this is a new method and
+  // adding a new pure method to an interface would be a breaking change (even
+  // though this is private and non-API)
+  virtual std::vector<
       std::unique_ptr<experimental::ServerInterceptorFactoryInterface>>*
   interceptor_creators() {
     return nullptr;

include/grpcpp/server.h

@@ -191,8 +191,7 @@ class Server : public ServerInterface, private GrpcLibraryCodegen {
   grpc_server* server() override { return server_; };
 
  private:
-  const std::vector<
+  std::vector<std::unique_ptr<experimental::ServerInterceptorFactoryInterface>>*
-      std::unique_ptr<experimental::ServerInterceptorFactoryInterface>>*
   interceptor_creators() override {
     return &interceptor_creators_;
   }