Rework QSerialPort::setWriteBufferSize() docs

Fix the wording as suggested by AI, and fix one actual mistake - when
talking about passing 0, the documentation was mentioning an input
buffer, not the write buffer. Change it.

Amends 07d21cb06765c31d3f7da39991d086dcd8719f26 (6.10)

Pick-to: 6.12
Change-Id: I95a2436d39893f007d077051f3064ea4b0ff8fd2
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>