Update the notice about native image installation

Author: steve-s

mx.fastr/mx_fastr_dists.py

@@ -310,7 +310,7 @@ def mx_register_dynamic_suite_constituents(register_project, register_distributi
 "   ${graalvm_home}/jre/languages/R/bin/install_r_native_image\n\n" +
 "The native image is then used by default. Pass '--jvm' flag to the R or Rscript launcher to " +
 "use JVM instead of the native image. Note that the native image is not stable yet and is intended for evaluation " +
-"purposes and experiments. Some features may not work when the native image is installed, most notably the --polyglot switch. " +
+"purposes and experiments. Some features may not work in the native image mode, most notably some Rcpp based packages. " +
 "The native image can be uninstalled using the installation script with 'uninstall' argument.\n\n" +
 "See http://www.graalvm.org/docs/reference-manual/languages/r for more."
 ))