qemu-options.hx: Improve -serial option documentation

The -serial option documentation is a bit brief about '-serial none'
and '-serial null'. In particular it's not very clear about the
difference between them, and it doesn't mention that it's up to
the machine model whether '-serial none' means "don't create the
serial port" or "don't wire the serial port up to anything".

Expand on these points.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20240122163607.459769-3-peter.maydell@linaro.org
(cherry picked from commit 747bfaf3a9)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

qemu-options.hx

@@ -3968,7 +3968,8 @@ SRST
     This option can be used several times to simulate up to 4 serial
     ports.
 
-    Use ``-serial none`` to disable all serial ports.
+    You can use ``-serial none`` to suppress the creation of default
+    serial devices.
 
     Available character devices are:
 
@@ -3990,10 +3991,17 @@ SRST
         [Linux only] Pseudo TTY (a new PTY is automatically allocated)
 
     ``none``
-        No device is allocated.
+        No device is allocated. Note that for machine types which
+        emulate systems where a serial device is always present in
+        real hardware, this may be equivalent to the ``null`` option,
+        in that the serial device is still present but all output
+        is discarded. For boards where the number of serial ports is
+        truly variable, this suppresses the creation of the device.
 
     ``null``
-        void device
+        A guest will see the UART or serial device as present in the
+        machine, but all output is discarded, and there is no input.
+        Conceptually equivalent to redirecting the output to ``/dev/null``.
 
     ``chardev:id``
         Use a named character device defined with the ``-chardev``