[Twisted-Python] pb.Copyable docs

[Jeff Hodges]

As a newb, I've found the howtos invaluable but I came across a problem
when I attempted to write code based on pb.Copyable class.  Some of the
code holds vaguely described variables such as "state" and
"perspective".  I've inferred a little bit from other bits of
documentation but can't seem to fit it all together.  This line was most
disconcerting to me:

"XXX: understand, then explain use of varying cached state depending
upon perspective."

Anyone help a poor newbie out? Thanks, folks.
	-- Jeff