[disc] cord type support

[Erik Ostrom <eostrom@research.att.com>]

| On receipt of this message, an implementation should check if the
| specified type is understood. The semantics of "is understood" are
| fuzzy and up to the implementation -- most implementations will
| presumably provide some way for code to indicate its support of a
| particular cord type. 

We tied cord names to protocol names, right?  (The cord named "foo-bar-baz" is 
necessarily a part of the protocol "foo-bar"--or the protocol "foo", maybe, 
this is unclear.)  I think what I wanted to imply by that was that protocol 
definitions may include cord types.  So, a cord is understood if the protocol 
to which it belongs is understood.  Right?

--Erik