Document that the web server needs to allow `keepalive`

We added:

Note that if the server is configured with keep-alive timeout, it’d need to be at least the same timeout as in the sbws HTTP requests, which is 10 seconds by default (http_timeout variable in the configuration file, see more about in the next section).

but we didn't add that it does actually need keepalive.

Probably we should document too that it needs to accept application/octet-stream and some TLS versions (the ones that python requests accept by default)