qemu-options.hx: Improve -serial option documentation

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 747bfaf3a9d2f3cd51674763dc1f7575100cd200)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>