qemu-options: mention C-a h in the -nographic doc

Otherwise, a new user will be wondering how to switch between the
console and monitor.

Cc: Anthony Liguori <aliguori@us.ibm.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: NRamkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

qemu-options.hx

@@ -844,7 +844,8 @@ you can totally disable graphical output so that QEMU is a simple
 command line application. The emulated serial port is redirected on
 the console and muxed with the monitor (unless redirected elsewhere
 explicitly). Therefore, you can still use QEMU to debug a Linux kernel
-with a serial console.
+with a serial console.  Use @key{C-a h} for help on switching between
+the console and monitor.
 ETEXI
 
 DEF("curses", 0, QEMU_OPTION_curses,