netdev tftp option docs don't mention that the TFTP server is read-only

In qemu-options.hx we document the 'tftp=dir' suboption of -netdev user. This creates a fake TFTP server that serves files to the guest from a local directory. The TFTP server (provided by libslirp) is read-only. We should mention this so that users aren't confused by trying to use the "write file" features of the TFTP protocol.