Add explanation for flush/reset etc in node help

[cymplecy]

Hi I hadn't used the node for a while and I got mentally confused as to what flush did - I just pictured all the messages being flushed away down a toilet :)

Took me a while to work out that I should be using reset, to delete all queued messages, instead :)

Would mind if I did a PR to add a bit of an explanation to the node help to save confusion in future? Simon

[colinl]

I just pictured all the messages being flushed away down a toilet :)

I think that is a perfect analogy for the flush operation. The drainpipe maps to the wire connecting to the next node. :) What it says about the quality of the messages waiting to be flushed though, I am not sure.

[drmibell]

A PR is always welcome -- especially for documentation. "Flush" is fairly standard, but not universal, terminology, for sending along the entire queue, unlike "purge" or "reset" for deleting it.

I have it on my list to do a full re-write of the README. New features and clarifications have made it a bit difficult to follow, but until I can get around to it, I won't mind another patch.