[NNTP] Article references

[Clive D.W. Feather]

Ruud H.G. van Tol said:
> I still see some problems. The "but is not required to be globally
> unique" is trying to counter a popular misconception, but is not
> adding much information.

After some thought I've done some rewriting of this paragraph. It now reads
as follows, which I think is rather clearer as to the intent.

    News reading clients have available a variety of mechanisms to
    retrieve articles via NNTP. The news articles are stored and indexed
    using three types of keys. The first type of key is the message-id
    of an article and is globally unique. The second type of key is
    composed of a newsgroup name and an article number within that
    newsgroup. On a particular server there MUST be only one article
    with a given number within any newsgroup and an article MUST NOT
    have two different numbers in the same newsgroup. An article can
    be cross-posted to multiple newsgroups, so there may be multiple
    keys that point to the same article on the same server; these MAY
    have different numbers in each newsgroup. However, this type of
    key is not required to be globally unique and so the same key MAY
    refer to different articles on different servers. The final type
    of key is the arrival timestamp, giving the time that the article
    arrived at the server.

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