[http] add note to THttpServer docu

(cherry picked from commit 2993ad8)

Author: linev

net/http/src/THttpServer.cxx

@@ -84,11 +84,13 @@ class THttpTimer : public TTimer {
    }
 };
 
-
-/** \class THttpServer
+/**
+\class THttpServer
 \ingroup http
-
-Online http server for arbitrary ROOT application
+\brief Online http server for arbitrary ROOT application
+\note This class provides HTTP access to ROOT objects. The user is entirely responsible for the security of the server.
+It is strongly recommended to use the server only within an isolated network
+or to enable proper authentication to prevent unauthorized remote access.
 
 Idea of THttpServer - provide remote http access to running
 ROOT application and enable HTML/JavaScript user interface.