docs/specs: Convert pci-serial.txt to rst

docs/specs: Convert pci-serial.txt to rst

Convert pci-serial.txt to reStructuredText. This includes
some wordsmithing, and the correction of the docs to note
that the Windows inf file includes 2x and 4x support
(as it has done since commit dc9528fdf9f61 in 2014).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230420160334.1048224-3-peter.maydell@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>