Document that gtk3 support is experimental

Change-Id: I87df8cb6a4e4e374c5c9782e695fcdf020d5707f
Reviewed-on: https://gerrit.libreoffice.org/12121Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>

configure.ac

@@ -1077,7 +1077,8 @@ AC_ARG_ENABLE(gtk,
 
 AC_ARG_ENABLE(gtk3,
     AS_HELP_STRING([--enable-gtk3],
-        [Determines whether to use Gtk+ 3.0 vclplug on platforms where Gtk+ 3.0 is available.]),
+        [Determines whether to use Gtk+ 3.0 vclplug on platforms where Gtk+ 3.0 is available.
+         This is experimental and may not work.]),
 ,enable_gtk3=no)
 
 AC_ARG_ENABLE(systray,