QSerialPort docs: clarify negative values in set{Read,Write}BufferSize

Generally speaking, passing negative value as a buffer size does not
make sense. However, in practice negative parameters are handled
differently for read and write buffers.
Document it to avoid confusion.

Task-number: QTBUG-144899
Pick-to: 6.12 6.11
Change-Id: Ic608ebb00b9fe44c1e53f61d9e7172907b9e3110
Reviewed-by: Tatiana Borisova <tatiana.borisova@qt.io>