doc: document that we need the O_NONBLOCK call

A large percentage of real-world use-cases are tied into some external
mainloop and require non-blocking operation. It's too easy to forget about
that, so let's add the O_NONBLOCK call to the example documentation here.
Those that don't need it can remove it easily anyway.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>