tor-proto: Add comments about buffer sizes to CC_XOFF_CLIENT

This tries to explain that the amount of incoming data we choose to buffer on an arti stream doesn't really matter for arti's socks proxy, since the amount of data buffered by the kernel is significantly higher.

I noticed that when pausing a curl download with ctrl-z, it would take several seconds for an XOFF to be sent, even though the transfer rate was >1 MB/s. So after looking at where the stream data is buffered, I found that the socket kernel buffers were much larger than I expected. But this makes sense because of tcp autotuning. Anyways I wanted to write this down somewhere.