ui/console: Note in '-display help' that some backends support suboptions

Currently '-display help' only prints the available backends. Some
of those backends support suboptions (e.g. '-display gtk,gl=on').
Mention that in the help output, and point the user to where they
might be able to find more information about the suboptions.
The new output looks like this:

  $ qemu-system-aarch64 -display help
  Available display backend types:
  none
  gtk
  sdl
  egl-headless
  curses
  spice-app
  dbus

  Some display backends support suboptions, which can be set with
     -display backend,option=value,option=value...
  For a short list of the suboptions for each display, see the top-level -help output; more detail is in the documentation.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20240731154136.3494621-4-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

ui/console.c

@@ -1632,4 +1632,9 @@ void qemu_display_help(void)
             printf("%s\n",  DisplayType_str(dpys[idx]->type));
         }
     }
+    printf("\n"
+           "Some display backends support suboptions, which can be set with\n"
+           "   -display backend,option=value,option=value...\n"
+           "For a short list of the suboptions for each display, see the "
+           "top-level -help output; more detail is in the documentation.\n");
 }