kist: Document why 8 cells in channel_should_write_to_kernel()

Comes from #40006 (closed). Simple comment to explain where that number 8 comes from. I got the information from pastly on IRC:

10:28 <+pastly> dgoulet: yes. I'm tempted to think there was something about 4k being a common default socket buffer size and that's the logic, but I'm not confident enough about remembering that being the logic. Especially since socket buffers grow.
10:28 <+pastly> so it just seemed reasonable and way better than 1