qemu-options.hx: Don't claim "-serial" has limit of 4 serial ports

qemu-options.hx: Don't claim "-serial" has limit of 4 serial ports

Before v2.12, the implementation of serial ports was limited to
a value of MAX_SERIAL_PORTS = 4. We now dynamically allocate
the data structures for serial ports, so this limit is no longer
present, but the documentation for the -serial options still reads:

 "This option can be used several times to simulate up to 4 serial ports."

Update to "This option can be used several times to simulate
multiple serial ports." to avoid misleading.

Signed-off-by: Steven Shen <steven.shen@jaguarmicro.com>
Message-id: 20240305013016.2268-1-steven.shen@jaguarmicro.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: tweaked commit message]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>