Changes

osmo_io: Improve osmo_iofd_notify_connected() documentation

The mentioned scenario happens when using osmo_stream_cli inside
libosmo-abis BTS code connecting to an osmo-bsc in the loopback
interface.
The client sends SYN and the server sends SYN+ACK and then quickly sends
an IPA ID GET.

Change-Id: Ica20a050b98d117995a5b625b23ab9faa61aabee