docs: UML: Mention telnetd for port channel

It is not obvious from the documentation that using the "port" channel
for the console requires telnetd to be installed (see port_connection()
in arch/um/drivers/port_user.c).  Mention this, and the fact that UML
will not boot until a client connects.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Acked-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>