Changes

cosmetic: stream_srv: srv_link: Document no need for OSMO_SOCK_F_NONBLOCK

osmo_fd_register() currently takes care of setting the socket as
non-blocking, before any blocking call is performed. Document it.

If we ever switch from osmo_fd to io_uring to process the accept() op
then we won't need to set the socket as nonblocking anyway, since
io_uring expects to work with blocking sockets.

Change-Id: I98a6ef55a0a7289b9eeb7e8062e9afa4d87f51eb