[rsyslog] output plugin calling interface

[Rainer Gerhards]

David,

I have updated

http://www.rsyslog.com/download/design.pdf

So the definitions should be more clear now. But I managed to edit only up to
section 2.6. 

Note that there is a new state diagram in 2.5, but limited description so
far. But I guess it is useful for you.

In 2.4, you find the definition of what I call the "at-risk-set" of messages.
These are the ones that are potentially lost during e.g. a power failure.
Note that that set is non-empty even if we have ultra-reliable queues,
because there exist some messages which were not yet enqueued (those that the
inputs are working on).

Rainer